@ Wokstation
Sorry, your statement is not generally applicable. In Australia, it is the photographer that has to be on the private property for the landowner to have rights to prohibit their actions. If you are taking photographs from outside the school grounds, there is nothing to stop you unless you are causing a nuisance (possibly if you're there every day) the images are indecent, or they're in a setting where the subject may have an expectation of privacy (e.g. in toilets, change rooms etc. Not just on private property which is in plain view of public areas.) I suspect this may actually be the case in other jurisdictions as well (but IANAL, YMMV, MTG). Mind you, the teachers have a legal duty to question your actions but, if you have a reasonable explanation, no right to stop you.