Stella II is an extremely easy to use object-oriented modeling environment, is widely used and documented, and there are many pre-made Stella models available on-line for science education. In addition, Stella II:

• Is intuitive to use and is mathematically rigorous.
• The interface level provides easy user input with dials, slide bars, or input boxes and graphical or numerical output tables.
• The structural diagram level shows qualitative model structure and images can be added to better link models to real world.
• The equation level shows detail of numerical difference equations
• Time step and numerical solution technique are controlled by modeler or user.
• Can be used by students to build their own models or to use and explore physical systems with pre-made models.
• Can be used to build models ranging from simple to fairly complex systems with many non-linear connections.
• Observational data can be graphed and compared with model predictions using Stella II.

ISEE Systems: Why STELLA for Education? (more info) .

Demo Dozen at Creative Learning Exchange has some good information regarding the philosophy and usefulness of modeling activities in education. Download the Demo Dozen package towards the bottom of this page and open intro with Stella II. A free demo version of Stella II is available at High Performance Systems, Inc.