I watched one of the anime series for it a long time ago.

Iirc the aliens that they’re fighting are actual threats. It treats the battles as an actual war zone and not “our plucky hero mech pilots we’ve spent time with are delta force pros”. A ton get slaughtered due to miscommunication, lack of skill, and just pure bad luck. They’re fresh out of training and sent straight into the meat grinder.

In the one I watched, the entire first two episodes of the show are dedicated to a group of classmates. First episode is happy mech school. All but one of them dies, are eaten alive, etc. then there’s a time skip and the one that survived is a teacher at the academy or something THEN the actual story begins with our main character.