You could say loitering? Here's the definition;
"stand or wait around without apparent purpose"
Another fitting word, which is also better, would be masquerading.
"be disguised or passed off as something else"
"You 'masqueraded' as a member of the athletic class."
This sentence works, because of whatever situation put you there, you still played along as one of the classmates. Best of luck!
Let me know if none of these words fit, I have others that can fit this.