The only one that really comes to mind is, well, Jim Kirk himself. He spent a fair chunk of his tenure as a cadet on the USS Republic instead of at the academy, and he managed to get field promotions to acting ensign and acting lieutenant. (The details of his academy days are kinda unclear and contradictory, since they changed things around between episodes, but this seems to be the generally accepted consensus according to Memory Alpha.) What all this means, of course, is that Jim Kirk surpassed Harry Kim before he even graduated.
About the cadets ask, I'm going through TNG rn and as far as I can tell, acting ensigns get credits for their practical experience in the field so it's probably like Work Study programs in college. But like the Ultimate Work Study program cause I cant think of anything cooler.
I always thought Wesley was just like a weird exception. I can’t think of any other acting ensigns, but I’m probably wrong?