Hey Curious D,
If uploading large files or often is a real concern, it seems that having aDSL is the better deal.
Typo? But yes, I think you're on the right track -- if you rarely/never perform large file uploads, then aDSL would be the better solution. I would imagine that being a elementary school, your main use of the Internet would be for web browsing (i.e. I don't think the kids would be doing too much video-conferencing, net-meetings, etc. -- activities which may place a large strain on your uploads). This is consistent with the sorts of activities a "home environment" would perform, suggesting that aDSL may be preferable for your setup.
Six times faster download speeds (or at least greater bandwidth), less expensive per month, nearly same upload speeds as sDSL (784 kps vs 1 Mps).
One of the main details I'd like to mention is that aDSL is a "best-effort" service, meaning that the advertised speeds will not always be achieved. sDSL performance is guaranteed. Since the advertised aDSL speeds are 6Mps, I'd think it would be safe to say you would always receive a better downstream connection than the sDSL, however how poorly your upstream would suffer is questionable.
Normally, the higher price tag on sDSL comes with certain service guarantees (SLA's).
The aDSL will provide 5 static IP addresses (is there an advantage to having many static IP addresses?) while the sDSL provides 1 static address.
This has me a little concerned. Certain ISP's may give you a set level of static IP's, and then cap you at that amount -- i.e. you can not use a router/switch to share your connection within a private network. This may mean you would not be able to share the Internet connection throughout the school. Before deciding on aDSL, I would strongly encourage that you get the specifics of why they are offering you 5 static IP's, and whether you can share the connection (without incurring greater costs).
Finally, if the school is hosting their own servers, etc. then sDSL would be the optimal solution. So, to answer your question -- it really depends on the types of activities the school uses the Internet for, as to which would be the optimal solution.