The opposite of 'attend' [closed]

There is no antonym for attend, because there are so many possible ways, and reasons, not to attend. It is not usually wise to assume that any word has a one-word antonym, for just this reasson.

Your edited question, though, is more reasonable: missing (a class) without justification could be shortened to skipping. I would, however, call this slightly too informal for an official university document, and think you would be better off spelling out 'missing 25% of lab sessions without good reason'.

Is the non-attendance of a delinquent nature, not merely a matter of choice? If so, you can say "cut", specially for classes. Not so much for a conference. Perhaps "skip" or "dodge" would do (at least in casual English). I doubt there is a single word for not attending where there is no expectation on you. Verbs don't necessarily have antonyms. "Do, or do not . . "

@TimLymington has some good suggestions, but based on your example, I would suggest breaking it up into two clauses.

• Students who fail to attend 50% or more of lab sessions will be ineligible to take the final. 25% of sessions may be missed for reasons out of your control (illness, etc.).

If you have eight sessions, I think this is clear a student who misses five, but was sick for two, is still eligible to take the final exam.

However, that's a lot of labs to miss. Most of my labs have been you can miss one, and extenuating circumstances are discussed individually.