I see many students resting their bell on their knee while they are playing. This is not a good idea for many reasons. And, yes, you may see professionals doing it, but that doesn't mean that it will help YOUR playing.
If your bell is on your knee while you are playing, then you are probably sitting with poor posture - slouching and/or pulling your head down. You probably already know that poor posture will affect your general health. As for your clarinet playing, poor posture will inhibit breathing and negatively affect embouchure, tone, hand position, etc. The bell on your knee can muffle the low E and third line B, and one of the low keys might even get stuck in your pants/skirt. If you become accustomed to playing with the bell on your knee, you won't be able to play standing up - for marching band, or solo performances.
My recommendation: sit forward on a flat seated, hard chair so that your back is not resting against the back of the chair. Keep your feet flat on the floor in front of you, your knees apart, and your head in a neutral position. Adjust the height of your music stand so that you don't have to tilt your head to see the music. Think of supporting the clarinet up into your mouth with your right thumb.
Here is a picture that demonstrates this.
Some clarinetists say that you have to rest your bell on your knee if you play with double lip embouchure. Since I play with single lip embouchure, I won't give my opinion on that. I use double lip embouchure as an exercise to "remind" my upper lip to engage.