Can a student stay in High school till the age of 20?

In most U.S. states, students can stay in high school until they turn 18. After that age, they are generally no longer eligible to attend unless they have an individualized education program (IEP) or a 504 plan in place that outlines specific educational needs requiring continued high school attendance. There are some exceptions and variations depending on state law and individual school district policies. So while 20 is uncommon, it's not entirely impossible under specific circumstances.