Cleansing is very important for a yoga practice, when you wanna go for yoga sessions must clean yourself inside and outside. inside cleansing you can practice some cleansing processes of yoga, if can't then at least clean your bowels, and go for class in empty stomach or after food minimum 3 hours and after finish your yoga practice don't eat any kind of solid food for 45 minutes, if you feel so hungry then can take yogurt or juice after 15-20 minutes. Don't take shower immediately after yoga class, wait for 30-45 minutes. For outside cleansing you can take a warm shower before the yoga session,wear clean and light color clothes, this will be very helpful to improve the level of your practice. after shower our body automatically warmed up, and blood circulation is running properly in whole body.if you don't have time to take shower then you can take Pancha Snana = Wash your five major parts) face, hands, feet before the yoga class.This is hygienic for you and for others. This will gives your pure and fresh feelings, then you can enjoy your practice in a better way. and also teacher need to touch your feet and correct your alignment during some Asanas, but if you don't wash then is not good for you, for your teacher and for other students, because teacher will use the same hands for others to correct so....
          and I saw some students even don't care to wash their feet due to hurry, and then bad smell of their feet influence the environment of whole class, even can't breath properly, if deep breath then all that bad smell is going inside of our body.So for the better and healthy practice try to take shower before the class, or take Pancha Snana (Wash your hands,feet,face).. Shower  need only 10-15 minutes, and Pacnha Snana just need 2-3 minutes.

Hari Om.

Namaste,
 Manu