Is expiation obligatory for one who swore to go to school without performing ghusl, intending not to perform ghusl rather than not to go to school, and then did not go?

The oath is based on the intention of the swearer. If he intends by his oath something that it can bear, his oath is directed to that, whether what he intended is consistent with the apparent meaning of the لفظ (wording) or contrary to it. So, if you intended to negate ghusl (ritual bathing) by your oath, you do not break your oath except by performing ghusl. And whether you went to school or not, no expiation for the oath is required of you.