You can also make banners out of felt for school colors. I don't know if I am misunderstanding you, but if I were invited to a party that was in a garage, I would feel as if I wasn't good enough to be entertained in the public rooms and would be very offended. It's all about celebrating a big milestone in your son's life. I would hate to be a guest where I would be considered an annoyance if I needed to actually enter the host's home. I feel the need to respond here to your feelings about having an open house in the garage.
Many people buy tents in case of inclement weather.
However, I have a small yard and no room for a tent. I do have decks and patios that will work if the weather is nice, including a covered back porch, it's just not conducive to a tent. I'm not trying to be rude but practical. Frankly I don't see anything wrong with this. I agree with some of the other girls about being offended.