I think it all boils down to TCM being such an informal group. Its not a college fraternity, there's no formal lists of members, no initiation rituals (forget the sex part, that's a rumor started by jocks to piss TCM girls off) and no official leader iirc.
So in this sense, Eric and Dylan were TCM, because it was a sort of open-membership thing. However I think its worth notijng taht they stuck to themselves and had little to none contact with most class of 1998 TCM people. So I'd call them TCM outliers or TCM associates, not to confuse things.
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"Bullet Time" - a school shooting film from Poland