Nesting gulls deterrent

I need to find a humane gull deterrent ahead of the nesting season next year. We are a secondary school based in the suburbs and the gulls nest on our roof. We have no issues with litter or food waste as that is cleared throughout the day but the gulls seem to have worked out that at lunch time we are a good source for dropped scraps of food. They aren’t a nuisance for the school but we receive lots of complaints from residents. I have seen silent ultra sonic birdscarers for sale for around £300. Are these effective and an acceptable way to deter the gulls.