Since the girls are still bringing the magazines in, I think it would be best to come up with another strategy. Some of the girls may even find it funny to watch you shred their magazines, hence keep bringing them in to antagonise you!
There are a few things I can think of that you may find useful:
1. Make the lesson as interactive as possible, to gain the attention of your students. Get the students doing activities where they need to walk around the room (e.g. work stations, solving problems on the white/blackboard, games that involve standing up etc)
2. You can actually incorporate fashion into the lessons! For example, you could give an assignment where the girls need to use algebra to solve fashion-related problems. The assignment could have a scenario about someone starting their own clothing business and needing to solve problems.
3. Use the magazines as a reward. Collect all the magazines at the start of the lesson. Explain to the girls they can have them back after a certain amount of time. Start with a short amount of time, and then increase this over the lessons.
Let me know if any of this is helpful! I hope it is. Good luck!