Are there any resources out there to continue developing my students' programming skills after Code Combat I am teaching several classes consisting of 12-15 year-olds the beginnings of programming by exposing them to Code Monkey and Code Combat. These resources successfully engage most of my students and follow a smooth progression such that most of my students are able to follow along mostly unaided. (There are significant differences in how fast different students go though the exercises, but that aside.)

My question is about similar follow-up resources: Are there any resources out there, for once my students have worked their way through Code Monkey and Code Combat, that will allow them to continue developing their programming skills, without the difficulty going up too much all at once (causing them to quit), but also without stagnating at the level of Code Combat?

To provide some context on what Code Monkey and Code Combat cover: some basic movement commands, basic looping, named variables, and that is pretty much it I think.
You could try the Bootstrap curriculum, the materials are free and can integrate with existing curriculum instead of being electives. A talk about the design of the program, and how to design highschool computer science curriculum in general is given here by one of the Co-directors of Bootstrap, Professor Shriram Krishnamurthi from Brown University.