Thanks for writing your query.
Urinary Tract Infection
(UTI) can develop even due to urinating in unhygeinic public toilets.
It might be a possibility that the school toilet your child is using is not clean.
Other causes of recurrent UTI can be :-
1. Renal stone
2. Holding the urine for long (it might be case in your child as some children donot go to toilets for long periods in schools due to either fear of asking permission from teacher or due to shyness)
resulting in incomplete evacuation of bladder
You should get an Ultrasound
of lower abdomen done with emphasis on KUB region to rule out any renal pathology.
Also have culture/senstivity test of urine done to know the causative organism and the appropiate antibiotic.
Also encourage him to go to toilets during school hours and not to hold urine for long periods.
I hope this is helpful to you.