I talk to some of my friends online every single day. We are literally on there for hours talking about the most personal stuff. But as soon as I see them in school my chest tightens, breathing becomes difficult and I can't say a single word. I want to spend time with them so badly but I am always afraid for no reason at all. 
Why is this so hard??

This is probably important information: they are all of the opposite gender and have anxiety too

Any advice would be greatly appreciated