Some school districts – typically those with traditional classroom function locksets – have set a policy to keep classroom doors locked at all times. There are also school districts that have installed storeroom function locks, which are always locked on the outside. The disadvantage of keeping the lever locked all the time is the inconvenience of having to open the door each time a student arrives late or returns from the bathroom.
Some teachers use a magnet over the strike, a commercial product, or a creative method to keep the door from latching under normal operation, to avoid the inconvenience of a locked door. I think this Fixed-it Friday photo is the most creative method I’ve seen: