I know I'm late to the party but Year 11 reports have kept me busy. I don't want to offer suggestions so much as to share a strategy. I arrived in a school that had lapsed into 4 hymn sandwich territory and also needed to introduce new material.
I decided on a 5 year plan, asking myself what I wanted the kids to have sung before they leave us. I find it takes the heat out of the urge to do everything today and allows things time to bed down. We change settings for whole school Masses after the first Mass of the new academic year and add things gradually, building up to the last Mass of one academic year and the first of the next. Masses with smaller groups may break with that year's pattern - usually as a result of there being kids in the group that are involved in the liturgical music in their parish.
On the grounds that we ought to be supporting the work of the parishes with the kids who go to Mass- and trying to make sure that the ones who don't would be more comfortable on the occasions that they do go (weddings, first communion etc) - I tried to find out what was being used in the parishes around us. Most of our parishes use Laudate and commonly Mass of Creation, Gathering Mass, Celtic Mass and the Farrell Euch Acclamations. These are the ones that I keep in mind as a kind of core repertoire, music that will be known by a significant number of children and teachers and music that the others might benefit from knowing if they occasionally attend Mass.
In addition to that I've set myself three other conditions. The first is that in any 5 year period we should have sung Kyrie, Sanctus and Agnus in Latin (/Greek) and to chant (I wouldn't mind also doing the chant Pater Noster before the next World Youth Day, but it's likely to be a step too far for some of my colleagues). My reasoning behind that is that the kids tend not to have learned an aversion to chant and Latin. The second condition is that, wherever possible, I include something (setting/hymn) that's been composed in the last several years. The copyright notices on our 'worship aids' include the year and I want the whole school community to know that liturgical music didn't stop in the 15th Century, or the 19th, or even after Pope John Paul II's visit in '82! The third condition is to be open to suggestions from staff and pupils - not generally for particular hymns as I don't want to encourage the idea that they should be chosen arbitrarily - that's how we ended up using the Canedo/Hurd Mass of Glory. It can be really easy to stay within our own comfort zone and I need to learn new music as much as anyone.
God willing, in a few year's time there'll be people in our parishes who are neither afraid of new music nor throw their hands up in despair when someone puts simple chants in front of them...Now that I've put into writing what I've been thinking for the last couple of years I can't help feeling that I ought to be sat here stroking a fluffy white cat - it wasn't meant to be a plan to take over the world