A pupil has brought a carton of 200 cigarettes into school and is selling them to classmates in Years 8 and 9.
You haven't seen the exchanges taking place, but you are reliably informed that the dodgy deals are going on right under your nose. You want to search the pupil's bag and locker, but you can't because that would be a breach of their human rights, so you would be breaking the law.
This is the situation many heads and teaching staff could face if legislation currently being debated in Parliament becomes law.
Last week, The TES reported that under the legislation schools would not have the power to search pupils - even if they were known to be carrying "violent" or "hardcore" pornography.
Many believe the Apprenticeships, Skills, Children and Learning Bill is a legislative hotchpotch being pushed through by the Government, spearheaded by the Department for Children, Schools and Families and the Department for Innovation, Universities and Skills.
Teachers' unions and opposition politicians have already called it a "bureaucratic muddle" and a "mixed bag", and there are concerns that it could leave teachers open to complaints and even lawsuits when it comes to the delicate matter of teachers' powers to search pupils.
The changes, proposed by Sir Alan Steer in his behaviour report and adopted by Schools Secretary Ed Balls, were intended to give teachers clearer guidelines on the power to search, but unions - particularly the Association of School and College Leaders - believe it could do just the opposite.
The ASCL has called the bill a display of "legislation hyperactivity". Anna Cole, its parliamentary specialist, said: "We just don't understand where this legislation (about the power to search pupils) has come from. It seems to have come out of the blue.
"Our concern is that this will protect teachers even less than the law does now. Where teachers assumed they had protection under common law, it now seems the Department is restricting teachers' protection to certain scenarios."
The main problem, according to the bill's critics, is that although it extends search powers for teachers by enabling them to search for weapons, alcohol, drugs and stolen goods, it means they are prohibited from searching for anything else.
The NUT, in particular, is at odds with the prescriptive list put forward in the bill. John Bangs, the union's head of education, believes government has its priorities wrong.
"The Department will not extend the search rights to include all things that could cause immediate harm because it would be breaching their human rights," he said.
"But health and safety should be the primary concern when it comes to imminent harm. We want to see a generic approach to the search powers.
"By making the list exclusive, you will automatically be leaving something out. It's like the classic case of throwing a party - the more people you invite, the more you offend by not inviting them."
Another proposed change in the law is that two staff members of each gender would have to accompany any school trip in case a search had to be conducted.
The ASCL has called the move "totally impractical" as it means four staff members would be needed on any trip, regardless of how few pupils were involved.
Ms Cole said: "This will just put the kibosh on any school trips. How many schools will be able to afford to go without four members of staff just for a small school trip?"
As the law stands, heads and staff can search pupils with their consent, and are allowed to force a search only when there is reasonable proof that a student is carrying an offensive weapon.
Peter Smith, an ex-police officer and former head of the Police National Search Training School in Kent, now trains school staff in searching pupils. He believes a prescriptive list could lead to difficulties.
"If you're going to draw up a list, it's going to be very long," he said. "Searches are generally spontaneous and I think it's unlikely a teacher is going to have a list on them at all times.
"If you are going to legislate, I think you should try and keep it simple. You need safeguards, but you can do that without all the whys and wherefores. People are in danger of not using their common sense.
"I suppose the safest thing to say is: if a member of staff is unsure, there is always the option of phoning the police."
The proposed changes to the law have been called "ill thought through" by the Liberal Democrats, who believe the legislation needs to find the "right balance" between highly prescribed search powers and legislation that is too open ended.
David Laws, the Lib Dems' education spokesman, said: "It is important to make sure there is more scope for allowing school leaders to use their professional judgment.
"A general power would be going too far, but we need to make sure that schools are confident that the law is on their side when safety is at at risk or a pupil is clearly in breach of school rules."
Nick Gibb, shadow schools minister, believes the bill is too specific on searches.
"We believe teachers should be able to search for any item that has been banned under the school's own rules, so that could include mobile phones," he said.
A DCSF spokesperson said: "Searching a pupil without their consent is a serious matter and not something that should be taken lightly.
"The new clause builds on current legislation around knives and weapons, extending the powers to cover three categories of items that under no circumstance should a pupil have with them at school and which threaten pupil safety - illegal drugs, alcohol and stolen property.
"It is important that these items are clearly specified in the legislation so that there is no doubt over schools' power to search, confiscate and dispose of them. A more general criterion would not make it sufficiently clear what kinds of items may, or may not, be searched for."
Conservatives and Liberal Democrats tabled amendments yesterday, but if the bill goes through as it stands teachers may have even more to think about next time they suspect a pupil of hawking cigarettes or pornography around the playground.
THE LAW AS IT STANDS
The Violent Crime Reduction Act 2006 gives heads and authorised staff statutory power to search pupils if they have a reasonable suspicion that they are carrying a knife or other offensive weapon.
- Searches must be in a private place. You must be the same sex as the pupil and accompanied by another adult of the same sex as the pupil.
- You cannot require the pupil to remove any clothing except outer wear, and a search of the pupil's possessions must be carried out in the presence of another adult.
- The search can be undertaken using "such force as is reasonable in the circumstances".
- If a search reveals offensive weapons or knives, or evidence in relation to an offence, the school must call in the police.
- Heads cannot demand that staff conduct searches - only authorise them to do so. Authorised staff should be properly trained.
- Since October 2006, schools can screen pupils at random for weapons using "arch" and "wand" metal detectors to protect pupils and staff from violent crime.