Something weird happens to my best friend in class! Is she doing this on purpose?

Sometimes in class, my best friend looks kind of diagonal up with her eyes and very slightly jerks her head sideways like people do sometimes when they are falling asleep, but the rest of her body doesn't move. Sometimes she doesn't blink for a long time. I look at her and ask her questions and even call her name but she doesn't answer me. It can last for a couple seconds to about ten seconds. After, she blinks and looks back at me as if it was the first time I have been trying to get her attention. It's so weird! Is she doing that on purpose to be annoying? I even pinched her once, but nothing happened until, many seconds later, she blinked, looked at me, then yelped as if I had just pinched her a second ago.I told her about it and she thinks I am making fun of her and claims she never zones out or daydreams, and then she changes the topic. Does she know she is doing this? I told her to talk to someone or even our biology teacher but she just laughs it off and says it never happens. Is this dangerous? Why doesn't she admit to doing it? I think maybe this is a really bad thing but she is scared of the hospital and doctors so she would never go check it out. I feel bad though because I think she doesn't do as well on tests because she never pays attention. What if this happens to her underwater or during something dangerous? Can she get hurt if she "zones out" underwater? Help this makes me nervous! I think some other friends see it too, but not the teacher. She is sixteen, this started to happen maybe a couple months ago. It happens maybe four times a day or more, I'm only in two of her classes.