There is no protocol, but all teachers are bound by the conditions of their contracts.
For teachers employed in state schools, the matter is dealt with in the "Burgundy Book", the agreement between the teaching unions and local authority employers. This sets out the period of notice that must be given for teachers, and the only three dates from which such notice must operate, namely October 31, February 28 and May 31. Heads are required to give at least one term's notice.
In independent schools, the terms of service will be set out in individual contracts, many of which are broadly similar to those in the state sector.
Notice should always be given in writing, although it is common practice for teachers to address letters to the head, rather than to the chairman of the governors or LEA. Unless the employer says otherwise, a resignation, once submitted, must be deemed to be accepted. It could only be withdrawn at the employer's discretion. Not every employer can be expected to be as charitable as Lord Palmerston, who was said to have a drawer where he kept the frequent resignation letters submitted by his chancellor, Mr Gladstone.
Do all classroom assistant posts need to be advertised, internally or externally?
Only head and deputy head posts must be advertised. For all other posts, the governing body is free to determine how to fill a vacancy. It is poor management practice to make appointments in such a way that people who would like to have been considered do not get the opportunity. This is particularly so when potential internal applicants are left feeling overlooked. There are times when a school wishes to offer a permanent post to someone who has done a first-class job on a temporary contract, when advertising, shortlisting and interviewing would be a waste of time.
Classroom assistants are more likely to be recruited locally than nationally and local advertising, though not obligatory, would often be the obvious route.