It wouldn’t stop a 0 from being entered as a grade — just not push that notification.

This would actually makes teaching easier, as they can use a 0 as a placeholder without having to decide about accidentally pushing a notification.

I don’t think the teacher’s goal is to cause a panic with a transient 0 that’s later replaced — so I don’t see why they’d stop using something that better accomplished their goals.

101 would be a relatively safe number to use as a placeholder. Since these people aren't programmers, trying to use NULL or NaN or -1 will probably backfire.

Sentry values are a nope. Have the assignment have a is_published check box / flag. An option to select all as published and the ability to toggle/select individual's assignments as published.

No, there could be extra credit