Same here we use LTO tapes to make an end of year backup that we keep indefinatly probably 10 years we also use an external HD when that is full we get another, at current sizes and prices it's worth doing.
Students can use examples of their A level work in their degree and so might come back to us for at least 5 years and while we might not be obligated to keep it it would be nice to have it.
We tend to give staff 'their' work when they leave on DVD and the rest will remain on tape when the end of year backup is run.