Parents never assume, but patronising know-alls running Inset courses sometimes rub this message home with a little mnemonic. Turning the word around, they say: "When you assume something without checking - U are making an ASS out of ME."
So, at the parents' evening, when you are faced with Mr and Mrs Smith, the obviously white parents of Nicky, who is black, don't jump to conclusions. "Oh," you say, "I didn't know Nicky was adopted." "She isn't," replies Mr Smith.
Similarly, don't visit the sins of the offspring upon the parents. Darryl may be an insufferable little toad given to behaviour that would have been seen as extreme by Attila the Hun - but his parents turn out to be devout Methodists.
And try to avoid the kind of first contact judgments that will get you into trouble. Ms Cherry is wearing a leopard print mini-dress and about a kilo of make-up. She's an older, larger version of her daughter, who does as little school work as possible and specialises in reducing men to jelly.
"I'm rather worried about Tara," you say. "Unless she buckles down to her studies she will find it very difficult to get a decent job when she leaves school."
"Tara has a modelling contract worth pound;20,000 a year," says Ms Cherry. "Over the summer, she worked in eight countries. At home, she's learning Italian with her dad. I'm concerned that the education you are offering isn't meeting her needs, which is why she isn't working as hard as she could."
Full-scale retreat is the best option here.
But the worst, the most embarrassing, assumption that you can make, is that you actually know who these people are. "He's doing really well," you say, heaping praise on the head of one of your brightest and most affable students.
The couple in front of you are clearly delighted by this news, and as they leave the mother begins to cry quietly. "Oh, thank you," she says. "I'm so pleased. He's been in such trouble with other teachers." She turns to the father. "I told you Darryl would come right in the end."