This sounds more natural to me. Depending on whether the context is formal or not, either one could work. However, the first one tends to limit the group members to those who are in the same class(same year), such as Class of '96, etc. and the second tends to limit the group only to those who are withing the persons circle of friends. I hope this info helps.

これも学校の同窓生という意味ですが、他にもナントカAlumniで、同じクラブとかを経験した仲間を指します。そういったひとつの組織の同窓という意味でなければ、friends, buddiesなどが適当かと思います。