Tyring to identify a 70s book about a teenaged girl in England, alone at boarding school over the holidays, who puts on a cape from the school theatre and time travels. She meets a boy in the woods from another era (Tudor?) who is whistling Greensleeves. He's involved in a plot of some sort. This is not Allison Uttley's book. Any ideas?
Time Tangle by Frances Eagar. 1976/7. The boy who sings "Greensleeves" is called Adam.
"While staying at her convent school over Christmas, a young girl meets a mysterious young man she believes is a ghost from the sixteenth century."