A call, an SMS or an email to say 'hi' is good enough. On the other hand, don't look over the many new people in your life who will make good friends if you give them a chance to. Another friend Derek graduated last June. Things were fine till he became a sudden recluse 4 months ago. This is the time when you are suddenly thrust into a new environment where you have to search for your life meaning and purpose.
Life is no longer just school and exams.
I miss school. At some point, you may even question the logic behind slaving away for the next 30 years, earning enough money for marriage, a flat, a car and your children's education. Except for a lucky few, the rest of us will probably still be searching for the elusive meaning of life years on. To know it is wonderful, but don't stress yourself out contemplating. Put your priorities in life straight.