Catherine Taylor, 46, was also found guilty at Aberdeen Sheriff Court last week of assaulting a girl who had lied about having eaten her lunch. She was cleared of eight other charges of assaulting pupils aged between four and 11 over a six-week period.
Sheriff Graham Buchanan said the headteacher's actions were "unnatural, improper and grossly excessive", and added: "This is an example where the accused lost all sense of propriety."
Sentence has been deferred until October 27 for background reports.
An Aberdeen City Council spokesman said that, in the light of the case, a new policy for reporting concerns about child protection had been implemented quickly at the school and all staff had since been given child protection training.