I started out with a leather kit from Amazon. It was like $140 & had all the basic tools to get started.
Plus it had 4 colors of thread, beeswax plus some other stuff.
I made a lot of stuff with that kit.
4 inch Square double layer coasters. Tool 1 side or both sides. Teach them stitch line layout, the border layout, then the stamp pattern layout. Then glue & stitch the double sided coaster together. Can even make a leather coaster holder.
I am going say, user error. You probably moved the piece when sewing or it slipped a bit while sewing.
I am no means a professional & I've only used my class 26 maybe 6hours total.
Maybe some more experienced pros will chime in.
I am going to recommend you get a nice wet stone & a good strop board to keep all your knives sharp.
Then spend some time practicing how to properly sharpen your tools.