Yearmate is the term used to describe members of the same "class" at Valdemar's collegia. As graduation is determined on an individual basis, according to the readiness of each candidate, a "class" is based on when each student arrived. Those who all arrived in the same year are called yearmates.
The shared experience of navigating their Collegium, while in most cases also being away from home for the first time in their lives, forges a tight bond of friendship between yearmates.
Heralds' Collegium yearmates are even closer than other students. Part of this is likely due to sharing the experiences of the Companion-bond and Gift training. Herald-trainees are also the only students who couldn't plan for attending the Collegium. They all had their lives upended by being Chosen. It is unsurprising that this would lead to extremely close friendships among Heraldic yearmates.