Being in charge of our Rotary Club's Youth Exchange programme, we deal with this often. Students must have a return ticket after the exchange year for visa purposes, but since there could be issues, the possibility of modifying the return is more than likely.

Our recommendation is to book the outbound (with a high level of certainty to dates and times) as they wish, then book the return on a fully flexible fare that it has no penalties for changes or cancellations. In that case, your return date is simply an approximation and unless there is a serious capacity issue, you can always change without penally.