Absolutely not okay. If you have proper study habits, and are organized, you would not need to skip Mass to “study”. As far as the Volunteer goes, why would you need to skip Mass? These are elementary school children. Either they have a CLOW service, where they are at the regular Mass for the Liturgy of the Eucharist (as you would also be), and are hearing their Liturgy of the Word in another venue. . .or, if they are in some sort of playroom or school setting DURING MASS, then you are with them in the playroom and attending Mass at ANOTHER time–like the Saturday vigil. Besides, many if not most college campuses offer Sunday evening Catholic Mass, even if the majority of the population is Protestant, too. If there is absolutely no option for you but the Mass time which conflicts with the “child volunteer time”, then you never should have volunteered in the first place. One good–the volunteering–doesn’t “substitute” for the other good–Mass attendance. One is voluntary, and one is REQUIRED.
As for the last part of your question: I don’t understand what you are asking. Are you asking if the parents should have the DRE fire students who don’t go to Mass?