Raspberry pi 3 (NOOBS) wifi connected and disconnected constantly!

Hi guys
i’m taking a class about Raspberry pi 3(NOOBS) in university in Korea
but most of students in the class have a problem with wifi connection.
we can see lots of wifi(AP) on the dev but when we try to connect wifi, it’s like connected and disconnected constantly. all of them!
and even when we try to do in our houses or different places, it’s same!!
How can we fix this problem??
please help us!!