I would say that 99% of them will flat out deny that you smell. If you put yourself in their shoes, you'll quickly realize that their line of thinking would be like, hmmmmm, I don't want to tell this person he/she smells because I don't know what he/she's going to do, maybe he/she will flip out and hurt me, or maybe he/she will breakdown and cry, in which case, I look like an insensitive bully. Even if I tell this person he/she smells, I don't have the expertise to fix it anyway, so maybe it's best to let a professional or someone close to him/her to help instead. This person might also be trying to entrap me into looking like a bigot/bias/unwelcoming/etc. And the list goes on.
If you're intent on approaching your teacher or anyone, make sure you are on good terms with them, but most importantly, explain to him/her in detail, your problem, your intention, and how their honest answer can help you fix your problem. Don't forget to reiterate that they will not be judged or persecuted for their answer and that you're relying on their help to do this.