if you can cast instant conjurations into the field from outside, you would be able to project them outward from the inside, no such thing as a one way forcefield.
Because magic doesn't work inside the field, but it does work outside the field. An evocation cast from outside the field (eg. fireball) would not penetrate the field because magic doesn't work inside the field. However, instant conjurations (eg. acid arrow) cast from outside the field work fine, because they are only magic as they are cast.
Acid arrow magically summons some ordinary acid. By the time it hits the field, it is just acid. You would need an anti-acid field (antacid field) to prevent the acid from damaging someone inside.
The caster inside the field cannot acid arrow out of the field, because he can't summon acid inside the field.