Well...that is the Gospel as preached. Jesus himself said he didn't come to bring peace, but a sword (Matthew 10:34). His message obviously still does that today, as this little event proves!
Maybe so, but the place for that information is in church, not at a public graduation. In a church, people come because they want to hear the word of God. At a graduation, people come to see their friends and relatives celebrate an accomplishment. It is appropriate to thank God in this setting, but entirely inappropriate to condemn those who are other religions. The message may not be wrong, but the venue certainly was. Imagine if you were a religious Jewish family who had come to celebrate your child’s graduation. How would you feel if the valedictorian told you and your family that you would all burn in hell?