For a while there in 2003/2004, we did quite a bit in Bioinformatics and one such event was the Biohackathon that was held with 40 top scientists from around the world. Some of those involved came from very distinguished institutes such as the Whitehead Institute where Jr and Ad had a chance to visit.
While doing sessions with secondary school students on Bioinformatics, one simple software was Cn3D. This is a helper application for your computer that allows you to view 3-dimensional structures.

The structure for the DNA (Double Helix)
You can download structures here from NCBI’s (National Center for Biotechnology Institute) website. This is a database website maintained by the NCBI and there are other databases you may be able to search. These are PRT file formats that can be used to show the structures. Cn3D simultaneously displays structure, sequence, and alignment, and now has powerful annotation and alignment editing features.
Students would use this to visualise the structures on each of the computers and teachers could explain the structures. You may be able to rotate the images and as a 3D representation gives a more realistic feel to how a virus is like, etc. Some teachers were providing classes with a series of structures where students had to identify them, analyse them with a given set of questions to answer.
The website has an excellent tutorial as well.
Download the software here.