Hello Joe,
First, make sure that your system is NOT proprietary. Your cameras should have BNC or RCA style jacks for video, and either bare wires or standard connectors for power.
A DVR would take the place of your multiplexer and your VCR. You should only have to disconnect your old equipment, then connect the cameras to your new DVR.
Considering that you are protecting a school, I would recommend an intelligent, yet easy to use system such as the PC-Witness Pro series DVR. To allow for easy future expansion, I would recommend the Pro V/RT model. In your case, I would recommend using about 5 - 12 frames per second per camera, which is more than fast enough to allow you to settle who started an altercation or to catch an intruder. Staying below 12fps will help save you HD space, while providing good high-quality motion video.
The
PC-Witness Pro or 1304NET may allow you an inexpensive and viable option.]PC-Witness Pro VRT[/url] can not only be set up to auto alert you via phone, fax, beeper, or email when motion is detected, but it can also detect suspicious objects, graffiti, and missing objects. In addition, it is part of a new meshed technology that allows you to utilize analog or IP cameras on a single system. Better yet, you can even tie in multiple DVRs at multiple schools so that they all display on a single monitor!
Be sure to let us know if you have any more questions.