If all you know is some basic Dreamweaver, you may struggle to develop a professional, comprehensive website for your school. May I ask what you teach and why you have been chosen for the job? With respect, it sounds as if you're not a web developer, so why have the school not gone for a web development company and/or professional team/individual?
What software you use to create the website depends on what the website is going to do. Anyway, I'd recommend using the Joomla! content management system for your website over any other CMS available, provided you're only going to need things like school information displayed, information about teachers/students/timetables/etc. Joomla! is easy-to-use, and can be learnt by the non-technical staff in a week or two with a little practice.
Can you provide more details?