(I know this sounds like a repeat question, but I am asking this specifically for a classroom context and I have some specific needs (other than "free") that I feel sets this question apart. Please do not close it without reading.)
I am teaching a very introductory course in interface design and user experience.
My students have used PowerPoint to create low-fidelity website wireframes and clickable prototypes. They are comfortable with this process, as they use PowerPoint quite a bit.
I am seeking a collection of stencils/images that can be easily placed in PowerPoint to allow them to create wireframes for BOTH iOS and Android applications. The wireframes will be the basis for clickable prototypes.
I understand that Google has produced images that can be used in high-fidelity mockups. However, we are working mostly with low-fi images (think: Balsamiq's sketchiness.) Even in low-fi mockups I think it is extremely important to start to note the differences between the platforms' layouts and UI elements.
We don't have access to $ for software (which is why we have been using PowerPoint.) If there are other tools that have free access for educational use, I need them to cover BOTH iOS and Android (myBalsamiq, while great in many respects, does not have Android UI elements.) The software must also allow students to export their prototypes (so they can be submitted for grading.) I haven't found a satisfactory tool that does this.
Does anyone have any ideas short of me just drawing the elements in PowerPoint myself?