Comic 1: "I Just Don't Test Well"
Internal, uncontrollable, stable
Calvin blames his failure on his innate inability to take tests. It's something he believes that he was born with, and he is certain that he cannot control it, and it will never go away.
Comic 2: "I forgot"
Internal, uncontrollable, unstable
People cannot control whether they forget something or not, it just happens to them randomly. Calvin avoids guilt by attributing his bad test score to something out of his control.
Comic 3: "If I get a bad grade, it'll be your fault"
External, uncontrollable, unstable
Calvin could have done any number of things to have avoided being in the position he finds himself in, but it is more comfortable to attribute his future bad grade to his mother's lack of support. She exists outside of him, he can't control her, and therefore he can avoid responsibility for himself.
Calvin displays all of the signs of learned helplessness and a fixed mindset. Calvin attributes most of his failures to uncontrollable, external, and stable factors; in other words, he believes himself to be completely at the whim of his circumstances. Calvin is an intelligent young man, but he believes himself to be incapable of learning. As such, he doesn't study, and shows very little interest in learning or persevering through difficult tasks. Additionally, he makes excuses so that he doesn't have to feel shame for his self-perceived unintelligence.