Our experience has been very different. Average project from Premiere Elements - > 1GB in size for a tiny video + source videos. Same project done in Serif, < 100Mb, including all the source files.
We have plenty of free space currently on our SAN's so file size's doesn't concern us as much, obviously this changes from school to school depending on your hardware so granted, something to consider before buying.
Having said that for a 3 - 5minute video (which our KS5 produce for media and film studies) the main file sizes to consider are the original video media rather than the Premiere project itself. The render files themselves can't be ignored admittedly but still nothing to worry about too much from my experience.
And our budget doesn't need to stretch to that. We own CS3 Design Premium, and Serif. Master suite for our site would be over £10k, as we have more than 500 PCs. We've not found anything that Adobe can do that Serif can't, that is relevant to the courses being taught.
Ultimately I'm not arguing the principal people are pushing about not teaching a software but teaching a skill but I am arguing that you can do both and give the students some better experience and end results.
My view may be tainted by the teaching staff within my own school and the frustrations with them but I will still stand by my point that in my opinion Serif isn't as good an overall software package as Adobe.