A The notion of an informal warning has no place in disciplinary procedures, which provide for oral, written and final written warnings as stages along the road to dismissal. Of course, not every mistake or act of misconduct merits formal discipline and your informal warning may be compared with the football referee who has a quiet word with a player, saying, in effect, that he had better be careful or he will get a yellow card. That is good management, but informal comments cannot be used as evidence in formal disciplinary procedures at a later date. For that reason, a teacher might reasonably object to a record of the incident being placed in his personal file.
There is, however, nothing to prevent you keeping a private note as a reinforcement of your memory so that, should further misconduct occur, you may come to the conclusion that a yellow card is called for.