Hehe, Keiichi-kun, you always stretch right after class ends, don't you.
Literal
Hehe, Keiichi-kun always class end-[when-たら], stretch do [right-よね].
ふふ is a soft feminine giggle. ノビする ('to stretch') uses 伸び ('extension') as a noun + する — a fixed phrase for the morning-wake-up kind of full-body stretch. 授業終わったら is casual contracted 授業が終わったら ('when class ends'), with the subject marker dropped. Final よね seeks confirmation with friendly intimacy. A playful, observational comment — something a close friend would make.