Your group leader sounds like she knows what she's talking about. Stick with it, weight loss is a slow, gradual, frustrating process. It is long term, not quick fix. There are so many little things that can effect the scale - time of month, drinking a glass of water (water is heavy!) whether or not you've pooped! (seriously, weigh yourself before and after a good trip to the bathroom!). There are WAY too many variables for one week to throw you into a tailspin.
During the 8 months it took me to lose 50 lbs, I had many weeks where the scale didn't move. Sometimes, several weeks in a row. After reaching 140 lbs, I stayed there for nearly 6 months before I lost more weight. 6 months!!! In my experience, it was perfectly normal. Here is a link to my weight loss chart for reference:
Just keep doing all the good things you are doing!
Be careful about eating - don't restrict so much you can't meet the caloric needs you are demanding with your workouts. 6 times a week x 1.5 hours a session is a LOT. Probably much more than is necessary. Eating too little can have long term, damaging effects on your metabolism that just make it harder to lose weight. Fuel your workouts and don't starve your body.