hello, i need to download unity version 3.1 of 4 (that is the version my school is using). Can i download older versions? If not, if I use a newer version at home will it transfer over to the older version without any problems?
AFAIK you can not open projects created on a newer version with an old one. Unity tells you so when you open old-version projects with the current version. I wonder why your school is using an old version? You will miss the fixes, and even if its a pro license i think you can upgrade for free in the same major version (3.x).
http://forum.unity3d.com/threads/20923-Old-versions-of-unity
You can get older versions of unity by typing the URL directly in your address bar:
For 3.5.1 MAC you can type: http://download.unity3d.com/download_unity/unity-3.5.1.dmg
As far as going from version to version, as long as you aren’t jumping big increments, Unity is generally fine. It will warn you if it needs to upgrade the project making it unusable in older versions.