Read this blog post, then go get a massage
As both a singer and a teacher, the one thing I find true of 99% of singers is that we use too much facial and jaw tension when we sing. As you already know, when the larynx is down, the breath is allowed to not only support, but to create sound by allowing the focal folds to vibrate properly. In contrast, when the larynx is up, the breath is not allowed to flow and the vocal folds are forced to slap together to produce sound. But how are the vocal folds slapping together? Because the breath isn't being allowed in, extra muscles and tissues are forcing the folds together. This is caused by using your jaw, face, neck, and other body tension to support your sound. Take a look in the mirror. Do you see your upper body being raised? Do you notice your head shaking or your jaw locked?
So now that we know the problem, how can we begin to address fixing it? Your teacher will give you plenty of exercises, all addressing the big issue: use the breath to support the sound. Each exercise is a good one and you should do them often. The only way to get rid of the bad habit is to create a good habit. But here's the problem: How do I know if my jaw and facial muscles are being used? I struggled with this a lot as a young singer (who am I kidding, I still struggle with it.....). I never knew what it felt like to be completely released in my jaw and face, so I didn't know when I was using the extra muscles to support my sound. It wasn't until I began getting massages regularly that I discoverd what the released sensations felt like. A massage therapist works the muscles in your body to release tightness and tension. The therapist also massages your facial muscles for the same reason. We carry a lot of stress and tension in our facial muscles which inhibit us from making supported sound with our breath.
My suggestion to you is to go see a massage therapist and ask for a facial massage (if you have the time and money, go for the full body). As the therapist is working out the tension, pay attention to the newly released sensations you are feeling. Your jaw is no longer clenched, your cheekbones aren't going through your eyebrows, and your neck isn't pushing through your larynx. Take this new released feeling to your practice room and see what happens when you sing. You will likely notice a lack of support. This is due to the usual lack of breath support and the now lacking extra-support you were receiving from your facial muscles and jaw. Take deeper breaths, make that air spin faster, use your breath to create the sound and leave those tension building muscles alone! (On a side note, your body is highly toxic after a massage. You will likely notice your voice sounding lower and may even have a headache from the toxins flowing through your body. Be sure to drink plenty of water to flush away those nasty toxins.)
As always, practice in a mirror. It's a dead giveaway for many of our mistakes. Happy practicing!!!