I think if they sue to get their diplomas, they will prevail. The school is affecting the rest of that student's ENTIRE LIFE *and* their ability to get a job and have a roof over their head/not starve. That is punishment FAR outside the realm of reasonbleness. Kicking out people who cheer is reasonable, even if it means stopping the ceremony. With-holding the diploma the student EARNED via meeting the criteria for it is not reasonable, and IMO illegal. (Even with the contract, signing something illegal like that doesn't necessarily give away your rights, and I'd say the contract is void because it is illegal, and unreasonably punitive.)
ETA: Are they asking the student, or the noise maker to do the community service? I could maybe see asking the noise maker to make amends somehow (so long as it doesn't demand they take off work) but asking the student to pay for their family member's "sins" isn't reasonable either.
ETA2: Also, Trent at the very end of the article has a valid point as well. With this now known, who is to say there won't be people shouting just to be spiteful when their name is called? Kick out the disruptors, or make them (if you can prove they are related to the student) do the community service. How about fining them instead?