My organization has taught more than 16 million kids to shoot 3-under in the past 15 years. We teach this because most newbies can keep the arrow on the arrow rest better that way. The string is easier for them to draw to anchor without twisting.
When teaching kids, if the arrow is falling off the rest, nearly 100% of the time they are shooting split finger, grip torque is the more minor cause.
Doesn't mean the arrow won't come off the rest with 3-under, just far more likely with split. Of course a newbie, of any age, is a youngster in archery, so what is good for the gosling is good for the goose.
If the student archer wants to switch to split after they've mastered arrow control, that's up to them. .Not right or wrong. Safety for everyone in the school archery class is the first priority even if "dear ole Dad" wants the youngster to shoot like he does.
I shoot just like the kids (be a hypocrite if I didn't) Wouldn't go back to split for money. I find it more comfortable and the bow can be made quiet enough.