SAN LUIS OBISPO, Calif. — As the delta variant of the coronavirus sweeps across the United States, a growing number of colleges and universities are requiring proof of COVID-19 vaccination for students to attend in-person classes. But the mandatory requirement has opened the door for those opposed to getting the vaccine to cheat the system, according to interviews with students, education and law enforcement officials.Across the internet, a cottage industry has sprung up to accommodate people who say they won’t get …