no, we wouldn't say 'interest is the best teacher' - it sounds strange and doesn't really make sense. Something like 'Being interested in what you're studying gives you the best chance of success' would be a good sentence.. (when teacher say something is 'ok', please ask them "would you say it like that?" The answer is NO!) :)

Thanks

"I have always believed that interest is the best teacher" would be more correct - it's an interesting idea too! Keep it up!