Witches Sacrifice Students on Island in 1969 or 1970 film

by Bubba
(Granbury, Texas, USA)

When I was just three or four years old my parents took me with them to the drive-in hoping I would fall asleep before the movie got too scary. No such luck. Now 40 years later I'm trying to find the name of that 1969 or 1970 horror movie.

The basic premise:

A college professor takes his class to a remote island for a field study. Something happens (someone gets hurt?) and half the class returns with the professor to civilization.

The rest stay for a weekend of fun but are instead picked off one by one by by a group of witches on the island.

I distinctly remember two scenes:

1. A girl is hanging upside down from a tree, decapitated or with her throat slit, and a bucket full to the brim is below her, with the last of her blood slowly dripping into the bucket.

2. A ritual scene is taking place with women in robes around a fire and a girl tied to a large 'X' cross being burned on her chest with an incense stick.

That's all I got. Any ideas?