Your school might not have a 3D printer yet, but your students can still create 3D models online with these free online tools.

123D Design from Autodesk allows you to create models by dragging pieces together online then sending them to 123D Make where your models can be printed. From those print-outs your students can construct their models. Both 123D Design and 123D Make can be used online, on iPads, and on Mac and Windows desktops.

3DTin is a free
web-based program for creating three dimensional models. 3DTin provides
templates that you can use to develop models or you can build a model
completely from scratch. The service is completely free to use as long
as you allow your models to be labeled with a Creative Commons license
and put in the 3DTin gallery. The gallery is a great place is to find
examples of what can be created in 3DTin. Models that you create can be
exported for use in other modeling software. Before you rush to try 3DTin
you should know that you do have to be using a modern web browser that
supports WebGL. For example, 3DTin will work with the latest versions of
Chrome and Firefox but will not work with older versions of Firefox or
Internet Explorer. 3DTin can be used as a Chrome Web App too.

Like 3D Tin, Tinkercad allows you to build 3D models within your web browser. Tinkercad provides pre-made pieces that you can assemble to build a
model. You can also select a model from the public gallery and edit it
in your Tinkercad account. If you create a free Tinkercad account you
can save your work online. The video below provides a great introduction
to Tinkercad.