A teep kick is used to keep distance, or to setup combos. There is more to a teep then kicking straight forward. You should ask your instructor how to execute a proper teep. There are probably tutorials on youtube about teeps.
when we were doing it, we did started out kick practicing and our instructor left. he had to help the gym head train. we were being watched by the other instructor, but he was doing a fitness kickboxing class. My partner who has been doing it for years didnt correct me when i did it, so i might have been doing it right, but just pictured it differently than what i was doing. And looking at this video
it looks like a more practiced version of what i was doing. what i was doing was using my whole foot instead of just the toes like in the videp