There are two sides to EVERY argument. That does not mean they're equally valid.
If by "right" you mean "justification", then no, they don't. The teacher is the wrongdoer here, not the ones who caught him at it. There is no valid justification for being as pissed at the LaClairs as at the teacher (and administration and school board).
And if by "right" you actually mean "right" (as in "inalienable rights", not as in "right vs. wrong", "turn right at the stop sign" or "right wing"), well, a right to get pissed does not equal a right to threaten. The ones making the threats are wrong. Not the ones targeted by them.