You don't say what the class is. If there is no "direct" physical contact, or a reason your friend's kids will be sharing hats, clothing, pillows, coats, sleeping mats, stuffed animals, laying or sitting on carpeting, or sharing combs, brushes, headbands, or sitting on cushioned chairs/furniture used by many, then chances are very low they'll (friend's kids) give lice to others. Lice is transferred primarily through direct contact and primarily they hide in fabrics and hair.
In short, you stand a greater chance of getting lice if your children go to her home for a visit and bring it home in their hair and clothes, than sharing a class with them where they are just sittting together in a room for activities.
Remember, you are probably coming in contact with lots of people out in the public who are infested with lice and other things much worse, unbeknownst to you but you don't catch it because you are not physically interacting with them.
True, movie theater seats are one the most popular way adults get lice, it is pretty rare. Lice is mostly a grade school problem because kids wear each others things, share brushes and combs, roll around in pillows, cushions, and rough house.
I say don't worry about her being there unless the kids are going to be that close where it's going to be a problem. I'd avoid going to her house and car(cushion seats ya know) for 3 weeks though. That's where you'll likely get "it."
One trick you can do to see if you have lice infested clothes etc. is to hang them in the bathroom and run the shower hot to get the steam going. Lice hate moist heat and will come out of the seams of the fabric items to seek relief.
