A Hamilton high school teacher has resigned from her job after she allegedly performed oral sex on a Year 13 boy the same night as his school leavers' dinner.

The dinner was held in November and the teacher allegedly performed the act on the boy the same night, but not at the venue.

The schoolboy complained to the school but no action was taken.

A source said the student then presented a sworn affidavit to the school board.

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The incident was witnessed by other students and it was widely discussed among senior students and on social media.

The teacher is now reportedly under investigation by the Education Council.

It is understood she went on sick leave at the end of 2015 and resigned this year.

The principal and board chair told Fairfax they could not comment on the matter, that it was inappropriate to do so while the investigation was ongoing.

Education Council spokesman Andrew Greig said the teacher "has undertaken not to teach" while she was being investigated.

"It would be improper and unfair to comment further because the investigation is active and we don't want to predetermine any outcomes."