I think it’s mean to let a kid go to middle school still believing in Santa. Other kids are not kind about this stuff, so I think it’s on the parents to tell them ahead of time. Don’t set your kid up to be bullied.
That said, every year my kids still play along and ask for gifts from Santa and sometimes even give little things from Santa. I’m in my 40’s and whenever my parents want to be especially generous, they still give me things from Santa.