I have no idea why the mass obligation was not stressed at your school. in our school the children go to Mass on every Holy Day when school is in session, and perhaps they don’t realize that opportunity has been provided because it is an obligation, because they also have a school Mass every Friday.
I know it is emphasized, because the principal repeats it every week, that they are still obligated to go to mass with their families on Sunday, but I see very few of these families at Sunday Mass. granted, some belong to other parishes, but I know when they come to me in 9th grade for Confirmation that fewer than half are attending Mass regularly (about the same as for CCD kids from public schools). Our pastor also makes the same point from the pulpit, especially when school parents are there. he just preached on the US bishops’ letter on who can receive communion, and of course nearly everyone went to communion, even though the holy day Masses were only half full.
It may be that it was taught in your school but not reinforced by your family practice, so the message never got through. don’t be too quick to blame the school. Did your parents make it a point to bring the family to Mass on days of obligation, to see that everyone had frequent opportunities for confession?