"my teacher George"
Here's a good explanation I found
The word مُعَلِّم = teacher/mentor. It is literally "the one that makes (smbd) learn". On the other hand, the word أُسْتَاذ = professor.
In practice, we use مُعَلِّم for kindergarten/primary school, and أُسْتَاذ for anything above high school (middle/high school is kind of a grey area). We also use مُعَلِّم outside the scope of school as just mentor. If you're familiar with the academic world, أُسْتَاذ is also used as part official titles (Assistant professor, Associate professor, Professor...)