The school should always be your first point of contact. You may do this in a number of ways:

Write an appropriate note or email to the relevant person(s) (e.g. classroom teacher, year level coordinator) outlining concerns.
Make an appointment to speak on the phone or in person with the relevant person(s).
Consider speaking with the school's student wellbeing leader if appropriate.
Arrange meeting times or phone calls through the school office.
Ensure the relevant person(s) is given a reasonable amount of time to take the steps required to resolve or address the concerns.

If the matter is not resolved satisfactorily, after discussing your concern with the school, you may wish to read the complaints policy for your diocese to see what options are available: