As long as it doesn't contradict your policy and procedures, seasonal employees should be allowed to go past the "deadline" set by your institution to complete the training. This does NOT mean that they don't have to complete the training, it just means that it's understood that they're away at school/unable to make it in for the training, and therefore will complete it as soon as they are available.
My institution is very big on retaining young talent and thus allows college students to take time off during the school year. That being said, the first day they come back on their breaks they're required to complete the same training as anyone else in their position. Scheduling is coordinated in order to facilitate this. Although they're not working for the same amount of time that doesn't change their exposure to the same types of risks.
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Life is like a bicycle. To keep your balance, you must keep moving.
-Albert Einstein
CAMS