A 'do' is just an informal word for an event or party... or even just a small 'get-together' with a few friends. It could be just about anything that people are invited to. So, .... he is leaving and has invited some people to celebrate - why not go if you have time and then you'll be able to say : "That was a great do." or "His do was really boring" or, if you are the only person there, you can say: "No-one came to his do!"

Leaving do is a small party for leaving a college.