Most likely, the teacher can't hear your specific thoughts, per say, but rest assured they can pick up when you are thinking, verses when you are completely absorbed in your breath/body awareness.
Can the teacher tell when you are having "unpure" thoughts?
But how? Do yoga teachers have special super powers?
Uh, no. Or, at least not this one. It is simple really. When in a good yoga class, the attention is inwards. The energy of the room shifts from the normal "mental/visual" level, lets call it, to a more inward/intuitive energy.
A good teacher knows inside and out the feeling of deep, one-pointed, non-judgmental, conscious awareness. Or more simply put, pure consciousness. While teaching, the teacher has "feelers" out, receptive to what is going on in their classroom. The teacher can feel when students are emitting that pure conscious energy. They can tell who is in their zone, and who is not.
Whenever a student is not reaching this potential, a different sort of energy, a tainted or "unpure" energy is emitted Aggression and ego have certain energy. Lust and envy have another energy. Distracted thoughts of past or future emit a sort of lower static or scattered-type energy.
The other day, I had a couple new students in my class who were in the front row. Try as I might to lead them to a quieter place within themselves, I could not over-ride their loud thoughts hurdling at me like poison darts as we moved through a variation of sun salutes. "What the hell is this? What the hell is that? Not this again! Why are we going so damn slow..." the thoughts kept hurdling.
After a while, the fresh prana starts to kick in. Their thoughts start to slow. My energetic shield lowers.
I find that any "unpure" thought in a yoga class environment is amplified. Even if the student is just thinking a mild, what the hell? The thought still has a direction (at the teacher, at themself, or a fellow student) and is still received as a poison dart. I bring this up to illustrate how powerful and harmful our thoughts are to ourselves and others.
Unpure thoughts are like poison darts. Poison darts people!
So remember this while practicing yoga. Stop hurting others and yourself by launching poison dart thoughts. Become aware when you are out of your zone, and take a deep breath. Refocus.
Even better. Remember your thoughts are powerful on and off the mat. Before you start launching your darts, take a breath. Find your heart center, think Cupid.
There is nothing sweeter than students emanating Cupid bliss arrows. They are not hurting anyone. They are melting the impurities and knots within, planting new seeds of pure-loving kindness in their bodies. They move from the heart and their light seems to fill up the room. They may even be helping to purify the energy of the classroom to help you back in your zone.