Surely, if the school are requiring the skirts to be a certain length and their nominated supplier only sell that length and it comes up a little short (for the school's requirement), then just tell the school that you have bought the longest length available in their nominated supplier. If the school don't like it, they will put pressure on the supplier to rectify it - it is not your responsibility to scurry around trying to find an acceptable alternative.
Looking at the uniform list on the website, it just says, this or this type of shirt and instructions about suitable fabric, not specifically that it has to be from Trutex, or have I missed something? I tried to have a look at the website of one of the suppliers for DD's school uniform to see whether they had an equivalent skirt, but it will only let me look at her school uniform (I don't think a green tartan kilt is much use to you).
I would suggest that the onus is on the school to suggest an alternative supplier.