Python provides an almost perfect interactive environment, just not the ability to save my work. For that I have created a workspace module that you may wish to play with.
I have used a version of this workspace twice in a summer institute for middle school teachers. Many of those attending were able to work through this lab with additional help from me. You may wish to try it as it provides a rather gentle introduction to Python.
A few others share my belief that Python is an excellent tool for developing mathematical maturity in high school students. Here is one of the first textbooks written in support of that belief.
Tcl/Tk for Programmers. The current version of Tcl is little changed from the one I covered in that book and the book is still in print (although it has changed publishers to John Wiley and Sons). For your perusal, about a fifth of the material has always been online.
After nine years in the same form, one might call
Tcl/Tk for Programmers
a classic. I won't because it has
not sold very many copies in any one year. The thing is it just keeps
on selling at the same rate.