where did OP say this was happening?
There’s a really fun and funny book about this called The Wednesday Wars by Gary Schmidt that I recommend to anyone interested in it. You need some massive suspension of disbelief about coincidences, but you will laugh.
When I was in elementary school in New Lothop, Michigan in about 1980 I remember at some point on some days catholic students went to catechism and protestants (myself included) went to something else where they talked to us about Jesus.
The “high Anglicans” in our school would get time off to attend 11 am services on weekdays such as Ash Wednesday and All Saints’ Day.
Jewish students would get a number of days off during the year. Hebrew school was always outside of school hours.
I think the way it’s done now across the board for Catholics is that the classes are either after school or in the evenings.
We didn’t have a “No Wednesday activities whatsoever” rule growing up in the 1970s, but activities and homework were minimized on that day. I’m actually OK with that; what’s wrong with an evening off if you’re a kid?
I’m guessing that the same rule applies to Mondays in heavily Mormon areas.
ETA: One wonders what day zombies might get off for “training.”