Here is something I learned as a meditation student. Start "meditating" while at home preparing to go to the class. If you change clothes, or comb your hair do it mindfully. Start "being in the class" while still at home getting ready to go. I used to do this when I first started meditating and it really helped. Of course I am sure once you are there you will be fine. You need to overcome the ruminations with mindful action.