I would give them a message on WhatsApp (or your app of choice) - and handle the situation like this:
Hey [name of friend], what's going on? It's been a while! Do you have time to grab lunch or coffee? I would like to catch up with you!
But of course, this all depends on your past relationship with the person you want to reconnect with.
In your situation - I would change the above approach to something like:
Hey there! [or the opening you're comfortable with]
It's been awhile! How is school going for you? Would you like to get lunch sometime? I have so much to tell you and catch up on!
And you would give a short snippet into how things were going in your life between then and now.
