Define your scope. The first step of any successful publication is to assess the needs of your audience. In this case, it's parents and teachers. Try to brainstorm some content ideas that will appeal to both groups. Regular features, such as a calendar of events and a message from the PTA president, can help personalize the newsletter and increase its usefulness.
Determine the frequency of your newsletter and develop an editorial calendar, so you can plan ahead for times when you have a lot to communicate, such as around graduation time. This may be determined in part by budget, especially if your newsletter is going to be printed and mailed, which can be expensive if you have a large distribution. If you are emailing the newsletter, you might be able to send it out more frequently.
Write thoughtful articles. Do your research, attend events that you are reporting on and personally interview people whose opinions and input matter. Be concise and clear in your writing. Chances are that most people only will skim the newsletter, so be up front about the most important points you want to convey. Bullet points and charts are great tools for grabbing the attention of your reader.
Edit and review; then edit again. Read over your articles carefully, not just for typos or grammatical errors, but to ensure that your writing is cohesive and to the point. A style guide will help you stay consistent with grammar, word use and punctuation--many editors keep a running list of style points that they constantly are updating as issues arise.
Seek out a second--and a third--set of eyes. Never trust just your own editing, especially of your own writing. Often when you are too close to a project, you will be more likely to overlook even glaring errors. So, have someone else review your copy before you finish your draft.