I have been involved with PTC in every capacity for the last 10 years and it is very hard sometimes to pull the parents in.
One thing we did that I feel like helped some was alternating meeting times. For example, Sept 3pm, August 6pm, October 3pm, Nov 6pm... etc. There were parents that were there for every single meeting no matter what time but it also gave other parents the opportunity to be there for at least some of the meetings.
Another thing we did was send out an email with the minutes.

Hope this helps.

Holding board meetings that way seems to be getting more common. Webcast general meetings are still pretty rare, I think.

As an alternative, some groups hold fewer general meetings and focus on getting parents involved through specific volunteering opportunities. That might work for you. Meetings are probably the least enticing thing parent groups do -- there's nothing interesting or exciting about going to a meeting, at least to people who aren't already dedicated to the organization. So just hold them once or twice a semester, say. You can conduct business at board meetings and find new officers by getting people more and more involved in volunteering.

I did see some information about virtual meetings on this site the other day. I definitely do think it's worth exploring. Have you used them for this type of group before?

Have you thought about having your meetings virtually? Perhaps via Skype /Facetime or Google Hangout? I conduct most of my meetings this way and find it very effective,

These days so much of what happens in meetings can be done from the convenience of your home or office - with some shared documents open in front of you.

Try a virtual meeting and see what happens. I think you might be surprised and how much people appreciate being able to participate and not having to drive to and from the meeting.