S-Rank Boukensha de Aru Ore no Musume-tachi wa Juudo no Father-con deshita - Vol. 11 Ch. 56

[Kreate]

That angle wouldn't fly at this point in time.
Since Etra used the same type of circle to erase the memories of the capital and it can easily be deduced that she doesn't need to keep providing any additional resources for that effect to keep running (given that she was away from the capital for so many years), it implies that, once the magic circle is done being cast, it becomes independent from the caster (maybe with the exception of the caster becoming deceased, not enough information to confirm that).
Therefore, rather than stating that the magic is still running (p.e. the brainwashing magic), it's more accurate to state that the effect is perpetuated until the caster decides to click the off switch.
Which leads to the arguable conclusion that the condition of "magic prohibition" would not suffice to cease the magic circle's effect, aka the barrier.

Not necessarily, the memory modification/erasure would only need to work for so long before it has achieved its purpose, its not a continuous effect. The barrier on the other hans continuously sepparates the academy from the rest of the world for as long as the magic takes effect, but no longer than that.

 

[Nautical]

Boring I thought she would just use brainwash on him or just brainwash all the students herself.