About the Pg. 26 TN note:
A
small tsu was probably used there. The small tsu (っ) is typically used to lengthen sounds if in front of another character (e.g. えち->えっち / echi -> ecchi) or denote an abrupt stop if placed after.
In this case, by itself it seems that was meant to be onomatopoeia intended to show him abruptly stopping whatever it is he was trying to say (due to pain), so something like "Ack" "Argh" "Khh" could work as translation.
It's different from the normal tsu (つ) which doesn't have a meaning of its own.
About the usage of "landmine", I think the closest thing in English would be the term "
loose cannon" since this girl is unpredictable to him and likes to live dangerously skipping classes so often.
If the source word translated into "landmine" was
地雷/じらい (jirai) I think definition #3 from Jisho could be the intended meaning. She's pretty (and sexy), but clearly a bad influence on him with a bad reputation to boot. "Landmine" with definition 3 makes sense to describe her from his POV.