As a teacher I tell parents never to sneak off, that promotes high levels of anxiety as the child can't afford to ever let you out of their sight as they can't "trust" you IYKWIM?? I know its hard (mine have all had their moments) but I usually call after about 15 mins and find they are fine. Some children just take longer than others to get used to CC, even DS goes through phases now and he has been going since he was 8 months!
It breaks your heart but you have to be strong to show your child that its a good place you are leaving them in and that you are happy about it.