Readings, Propers and themes.The church has invested an incredible amount of energy in selecting the combinations of Scripture readings, right down to the individual verses, specified for each liturgy. The first and Gospel readings are always linked to each other, they emphasise the message of the the feast or season - and the Sunday readings include some multi-week messages which build on each other eg the emphasis on light in the early Ordinary Time Sundays of Year A, and the Bread-of-Life series during the mid part of Ordinary Time in Year A
To say that these don't provide the basis for a theme of that day's Mass is just bizarre.
I am totally aware that the church has also chosen some small snippets of scripture, mainly from the Psalms, to use at transition moments in the liturgy. And that sometimes these snippets can be enlightening - especially for a musician or technician who misses the main message of the longer Gospel reading because they were distracted by how the sound system is working or how to support the soloist who's singing next or whatever.
But they are just snippets. They don't present the full story, except to an incredibly well-educated congregation for whom the smallest mention evokes a memory of the whole psalm and the way it's reflected in Jesus life . They mostly don't quote Jesus words. They're the Old Testament - not the Gospels, ie the Good News of resurrection.
Significantly, when Vatican II required the use of "the vernacular" (ie languanges that people use every day), the Church did not see providing translations of these snippets as an urgent priority. The focus was on translating the ordinary liturgical texts, and on exposing people to a wider range of content from sacred scripture.
Despite this, some people claim that these snippets (technically called "Propers", as opposed to the "Ordinary" ie the pieces that are the same at every Mass) should form the backbone of the music content at Masses in general, and including at school masses - where most of the attendees (where I live anyway) will be parents and children who are rarely at church anyway, not the most faithful and devoted (whose children are generally thoroughly grown and left school).
The action at every single Mass is the prayerful remembering and reliving mystery of the death and resurrection of Christ. But that's not the end of the story. It's just the start. And because it's ever present, it doesn't give particular guidance for how to choose materials for any specific liturgy. That's where the particular message from the readings, AKA the theme kicks in. Which is handly becuase it's through the scriptural and teaching messages of each day's readings that liturgy informs and inspires the people present, and gives them hope of sharing in the mystery of resurrection. Music needs to re-enforce and support all these messages, not just focus on Body-and-Blood, or resurrections - or other tangential ideas from the psalms.
Parish music directors and school massesAs to parish music directors not having influence at school masses, having read some of their stories of woe on the Catholic blogsphere, it's pretty clear that this is due to the behaviours of those music directors, and their often-astounding ignorance about education and human relationships.
So - some suggestions for the parish music director who wants to grow some influence in their school masses:
Firstly, learn some humility: the teachers may be liturgically ignorant, but you are pedagocially ignorant, and you aren't responsibly for the academic, social and spiritual development of 20/30 children five days a week. Their job is harder than yours, hands down.
Second, find out about the curriculum and teaching resources which the teachers have to work with. Mostly, they don't get to choose what ideas to present and what songs to use to support those. Probably they're working with their own musical limitations: if they choice is whatever they have a CD for vs no music whatsoever, then be grateful for the CD no matter how tinny it sounds to you.
Third, focus on building loving relationships, not liturgical correctness. If you're asked to help, choose mostly materials that the children do know - not just what you think they should know. Get over your pride and play those pieces which the education professionals know that the children and their marginally-churched parents will relate to, even if you hate them. Do it with a gracious attitude - make sure that you're modelling Christ's love in every single interaction that you have with the teachers and the kids. Be kind. Praise people for things that went well. Focus on the skills and talents that you have, not the defects.
Fourth, get over the excuses. Yes, the teachers work in the daytime, when most musical directors are at their day jobs - so take a half-days leave when you need to meet them. Yes, the children practise during school time, when you're probably not around - so provide resources that the teachers can use. Yes, most of them aren't getting the quality of musical education that you would like - get over it, and concentrate on what you can share with them.
If you do these things, chances are you will get to spend more time with teachers, and their pupils. You will be present when a teachable moment for making an liturgical point arises. And your voice will be listened to because you've made helpful suggestions in the past.