General:
* Teacher
* Trainer
* Educator
* Coach
* Mentor
* Tutor
* Professor (for higher education)
More specific:
* For technical skills: Facilitator, guide, practitioner, specialist
* For physical activities: Coach, trainer, drill sergeant
* For academic subjects: Lecturer, professor, adjunct professor, docent
The best synonym will depend on the specific situation and the level of formality required.