The downside of this is the pressure to find something for all the kids to 'do' in the Mass.
Fake action at school masses
In Australia, unfortunately, the stock standard approach seems to be to have a Gospel procession with kids clutching numerous streamers, a little liturgical twirling/dancing from the embarrassed looking girls, and mumbled contributions to the prayers of the faithful/readings.
Time for a rethink, because Rorate Caeli has just published a dubium from the Congregation of Divine Rites which states:
"The liturgical law of the Roman Rite does not foresee the use of dance or drama within the Sacred Liturgy, unless particular legislation has been enacted by the Bishops' Conference and confirmed by the Holy See. Any other practice is to be considered an abuse."
Genuine active engagement
Now personally I think there are more traditional solutions available.
First make the kids the choir (or at least the choir for a few items) for the occasion and teach them some chant or other simple traditional pieces to perform. Mulier Fortis offers a nice example of such active engagement in her own parish that shows that it can be done!
Secondly, have the kids prepare the Church for the event: perhaps they could help make a nice altar cloth in the appropriate liturgical colour for the day; decorate the Church with icons from home to decorate the bare walls of the Church (or even make some of their own under guidance) and flowers; polish the brass and so forth.
Thirdly, for those who really can't sing or play (I'm not suggesting guitar twanging, but some suitable pieces with instrumental accompaniment such as recorders could be considered for the Offertory), have a procession of half a dozen (boy) torchbearers (assuming you can rustle up enough candleholders!).
Fourthly, use the kids to teach their parents (and teachers!) to follow the actual rubrics when it comes to making the appropriate gestures at the confiteor, Creed and so forth.
Fifthly, ensure the kids understand that true active participation is above all about internal engagement with what is happening, not just external activity.
Finally, have the kids look distinctive and appropriately modestly dressed - perhaps wearing their school uniforms - as they perform tasks like take up the collection, do the offertory procession, act as welcomers and so forth.