It's out of the bizarre fear that they might let their friends get high off of them or something.
And yes, there have been multiple deaths caused by these policies. Once such incident was in Ontario in 2012, where a twelve year old died while he desperately tried to get to the office his inhaler was locked in. He kept trying to smuggle an inhaler in his bag and every time he got caught they confiscated it and made his mother come in to get it.
In 2015, a 14 year old girl died because her coach refused to let her leave PE to retrieve her inhaler. That one was local to me. In this case she had been forced to leave it in her locker instead of keeping it nearby.
All 50 states and DC now have laws requiring schools to allow children to carry their inhalers with them. I believe all Canadian provinces do too.
But there are still occasional incidents of teachers confiscating inhalers.
SOME states also have laws allowing school nurses to stock albuterol rescue inhalers for kids who forget theirs and some parents like to give the teachers or school a backup inhaler for the same reason.
But no, it used to be common, and kids died...and now it is a LOT rarer.