If you're looking for a formal adjective to describe something sly or secret, sneak in furtive. Let's hope the teacher doesn't see your furtive attempts to pass notes in class!
The adjective, furtive, is related to fūrtum, the Latin word for theft or robbery. This is apparent as the expressions "to give someone a furtive glance" and "to steal a glance at someone" mean the same thing. If a person's manner is furtive, he or she is acting suspiciously. Secret, stealthy and sly are all similar in meaning, but they lack this image of a thief's actions.