I'd love to answer but I have the same problem it's not the school saying we need sleeves but my dad. I can't find any cute dresses that I like with sleeves. I tried telling him about the sweater idea and he said yeah but you can take that off. Although I have found a few on deb.com that you can order online if you don't have a store near you. Any other websites or stores would be great. I need all the help I can get. Oh here's the link for a few great dresses on debs site
http://www.debshops.com/product/index.jsp?productId=4247951
http://www.debshops.com/product/index.jsp?productId=4247952&cp=3371579.3364605
http://www.debshops.com/product/index.jsp?productId=4073018&cp=3371579.3364605
http://www.debshops.com/product/index.jsp?productId=4066926&cp=3371579.3364605
http://www.debshops.com/product/index.jsp?productId=4100670&cp=3371579.3364605 IF YOU CAN DO ONE SHOULDER SLEEVES Here's ANOTHER LINK HOPE THIS HELPS. I STILL SIGHTS THAT HAVE COVERED SHOULDERS DRESSES... SLEEVES
http://www.debshops.com/search/index.jsp?page=1&kwCatId=&kw=one%20shoulder%20dresses&origkw=one%20shoulder%20dresses&sr=1

Yo ucould always have a shawl or wrap for your shoulders.

Depending on your age,finding a dress for your body type is important too,just because its in fashion,does not mean it will suit you,try on different styles with wraps,shawls or pashmina's. (sp)