it is more desirable to sing the responsorial psalm than to recite them

i've never read this, and i've read pretty much every church document on liturgy/music...can you provide a source for this? sacrosanctum concilium is pretty clear that liturgical norms are there for a reason...

Bruce, I reread your first post more carefully and see you are talking about replacing Psalm 118 with Psalm 122, as the response to Isaiah 26. (I had thought you were talking about using Psalm 122 for the feast of Saint Andrew.)

I am now even more comfortable with the recommendation I made to you because of the theme of the Isaiah reading. Psalm 122 would be entirely appropriate. Or, if your school children know a good Alleluia, they could sing it as the refrain to Psalm 118, which is an alleluia psalm. They could sing the response, even if the psalm were to be read.

That said, I cite for you and for BachLover2 Article 89 of the 1981 Lectionary for Mass Introduction:

Among the chants between the readings, the psalm which follows the first reading is of great importance. As a rule the psalm to be used is the one assigned to the reading. But in the case of readings for the Common of Saints, ritual Masses, Masses for various needs and occasions, votive Masses, and Masses for the dead the choice is left up to the priest celebrating. He will base his choice on the principle of the pastoral benefit of those present.

But to make it easier for the people to join in the response to the psalm, the Order of Readings lists certain other texts of psalms and responses that have been chosen according to the various seasons or classes of Saints [See Lectionary, ##173-174]. Whenever the psalm is sung, these texts may replace the text corresponding to the reading.

The first criterion of the priest's choice of another psalm is the pastoral benefit of the school children. (I hope he concurs with your substitution; be careful he is not preaching on Psalm 118!) The second criterion is the church's preference for the singing of the responsorial psalm or, at the very least, the response.

Like BachLover 2 I have studied all the magisterial documents on liturgy and music and I am well aware of the reasons for liturgical norms. The reason for this liturgical norm is that there is a preference for singing psalms over reciting psalms. The reason is enshrined in GIRM 61:

In the dioceses of the United States of America, the following may also be sung in place of the Psalm assigned in the Lectionary for Mass: either the proper or seasonal antiphon and Psalm from the Lectionary, as found either in the Roman Gradual or Simple Gradual or in another musical setting; or an antiphon and Psalm from another collection of the psalms and antiphons, including psalms arranged in metrical form, providing that they have been approved by the United States Conference of Catholic Bishops or the Diocesan Bishop. Songs or hymns may not be used in place of the responsorial Psalm.

Note that GIRM 61 does NOT say: The following may also be RECITED in place of the Psalm assigned . . .

I suppose this could be clearer, but it is clear enough for me.