I don't think any honest person, if they hadn't had the school previously pointed out to them, could reasonably claim that they could point out which building here is a school. As far as I can tell, there are no international symbols to show that a building is a school, likely because usually things go the opposite of how they went in Iran (this is all assuming it was in fact a school at the time of the bombing), typically a military will occupy a school to use as a base, rather than convert a part of a base to a school. If people aren't stupid or perfidious enough to convert parts of military bases to schools, a school will usually be very obviously a school with playgrounds etc. Now, even if there is no international symbol for it, you could certainly come up with some way to indicate that a building is a school if you wanted to right? That is, if you didn't want it to be bombed? Just painting the word "School" on the roof, regardless of language, would be enough. No such effort was made by the Iranians. Here is the "evidence" from satellite images that it was a school:
