@Rukja canonically gates leak mana and the stronger the gate the more mana it leaks, there does seem to be latent mana in the world between what leaks out of gates and possibly the awakened themselves leaking some (not canon, but probable), because of the first two points I believe higher rank gates can be detected from a distance and because of my theory about latent mana perhaps lower ranks can't since the little mana they do let out gets masked by said latent mana, so anything big enough to be dangerous to the world should be found out pretty much right away, whereas this thing is probably just a low rank gate...though these students are screwed unless one of them is a young "awakened"
also I'm sure it'd be bad for society as a whole if a bunch of orcs and goblins start pouring out into the middle of a city even if hunters can deal with them easily enough once they actually find them...as for gates like that that pop up in the middle of nowhere instead and don't get found...well, it's probably not that big of a deal if no one is around to be hurt by it? and once it is found, it'd get called in and hunted down asap
so if I'm right, unnoticed gates would only really be dangerous to normies who stumble upon them, or if like this one they happen to open in the middle of a city