It's not an idiom because it means just what it seems to mean. You should stay on the side of the fence that you are currently on and not climb over.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
Stay calm
An idiom is a saying or expression. There are many idioms that mean to stay away from, or avoid, someone. An example of such an idiom would be, "to steer clear of" someone.
To stay in touch is to continue to write/text/call or otherwise stay in contact with someone.
stay on her good side
This is not an idiom. It means just what it sounds like -- remain together in a group instead of separating.
the fence surrounds the school to make sure that the students stay inside the boundraries of the school
it means to stay in one particular area, not doing anything in particular. Often, you "hang around" with your friends.
Keeping your fingers crossed is a gesture that means you are hopeful about something. It means "stay hopeful that it will occur as you wish it to."
It means stay with the rest of the group.
They need to be tie on a fence
DO NOT CROSS STAY ON YOUR SIDE