I believe Eglamore and Parley went to Romania for some of Parley's training, and to have her meet the Rogat Orjak.
Gunnerkrigg Court is, any possible dimension-warping inside-is-bigger-than-outside heebiejeebery aside, somewhere in Britain. As is Zimmingham.
Paz had a visit home to Galicia (the one in northwest Spain, not the one currently divided between Poland and Ukraine, and not Galatia which is in Turkey), which was the other known trip off the island by Gunnerkrigg's current crop of students. Kat and family went to Scotland. Other than that, we know that most other students who aren't converted animals/fairies go somewhere for the summer but have no details.