Where are the Foreign National Military Students?

[ArmySGT.]

That makes the assumption that "the frontline is everywhere".
During the final years of the Twilight War that might be true but I don't recall anything in the timeline (to be fair, it's been a while since I read it) that indicates this is the case during the early years of the war.

This would be more true in the early and especially middle years. At that time bombers and conventional bombs; more so bombers with air launched cruise missiles are still available. Training bases are primary muster locations for reserve units and as such legitimate military targets that disrupt operations and demoralize your opponent when attacked.

Enemy training areas are typically low priority targets when you have enemy bomber bases and enemy theatre capable missiles to worry about.
Aside from all that, if we follow the line of reasoning that the attacker will attack training areas within the country, why would they not also attack airports and seaports to prevent those trainees from leaving (or re-entering once they've been trained)

I agree any competent attacker would; striking an airport or seaport could make them unsuitable for weeks or months. A strike on a training base is two fold. One you have killed or wounded the trainees, they are no longer fit to fight. You have destroyed the capability to train them, the instructors are dead or wounded, and the facilities smashed. Third, you have demoralized the local civilian population. Who is going to let their son or daughter sign up to a military that cannot even protect itself at home

It is the same reason the U.S. and Allied nations expend Tomahawks on desert encampments of tents.