In the States "Happy Holidays" has usurped "Merry Christmas" and is only used in that context.
If you are at the end of classes you might say to your classmates "I hope you enjoy your summer vacation" or "Have fun this summer".
A holiday in the States is a specific event, Christmas, New Year's Day, etc. A vacation is a specific time away. Summer vacation or summer break is the month or two between semesters.