I would go with invisibility. Time travel can mess stuff up.
EDIT: I'm glad this is back. Many memories in this thread.
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Edited by S7ven : 10/18/2014 1:57:47 AMInvisibility...you can -blam!- with teachers in school xD Edit: Shit that looks like I meant you can [i]f[b]u[/b]ck[/i] the teachers which I didn't I meant you can mess with stuff in the classroom to spook them.