18 year-old student Peter Butera was also senior class president at the school where he was giving the speech.
You can see what happened here, a minute or so in:
Apparently school officials didn't take kindly to being criticized at the graduation ceremony. Clearly many in the crowd weren't pleased with their decision to cut him off.
According to The Citizen's Voice:
The valedictorian and class president of Wyoming Area’s Class of 2017 had his microphone silenced mid-speech during Friday’s graduation ceremony when he started to criticize school administrators.
Thankfully, we're now able to see what Peter had planned to say in its entirety: