"I have gone to X school since I was 15" should mean that from the time he was 15 to the present he has made trips to the school. I'd say stay away from it because it is not a very concise or perspicuous idea, unless all your student has done is simple make a journey to the school.
I think it's easier and more accurate to just use the verb attend:
I have been attending X school since I was 15

Don't use the contraction, otherwise got really should be used with it, else it's awkward.

It is only in the abnormal cases where English words taken directly from the Latin are inflexive when representing number (id est singular or plural). Such words are generally taken from science, such as bacterium(sin.)/bacteria(pl.). I find it to be quite pedantic to decline all latin derived nouns which retain the same form. As far as grammatically correct English goes the Latin inflexive changes and the standard English pluralization are generally both accepted, exempli gratia, octopus can be either octopi or octopuses. It really comes down to the fact that English is not Latin and therefore should follow standard English rules first, unless for words that have special exception such as bacterium.