If you wish to have a procession to begin the liturgy, a suitable hymn is needed. Alternatively, you could begin simply with the priest, or other presider, in place. Again, a hymn may pull the community together. But starting in silence is also effective and different from the usual liturgy.
If you have a procession, you may wish to bring up the seven lit candles and place them on the front of the altar. Or they can already be in place. It is suggested you use large candles (of the IKEA sort).
The introduction is important. It could be read by one or several readers. Read slowly and reflectively. It is designed to focus attention and create calm.
After each reading, a pupil should come forward and extinguish one candle. This needs to be given some thought so it is done in a dignified and dramatic way (using a snuffer rather than blowing it out perhaps). It is also an idea to have all the lights on at the start of Tenebrae and then to put some off as each candle is extinguished until there are no lights on when the seventh candle is hidden.
By tradition, the seventh candle (representing Christ, light of the world) is not extinguished but is solemnly taken from its place and hidden (usually behind the altar). This should be done in silence and followed by silence. It is suggested there are no further prayers, hymns, or blessings. Pupils are dismissed in silence (and asked to keep silence until they are outside the church, or wherever) - if this is explained in the homily it can be very effective. Supervising staff need to be briefed so that they know how to dismiss pupils without speaking and are themselves silent and solemn.
It is traditional to pause after the words of the fifth reading "Then Jesus cried again with a loud voice and breathed his last." Again pupils need warning that this will happen and why.
The sixth reading is the dramatic portrait of the suffering servant from Isaiah. It is intended to provide a good lead into the homily or reflection which, it is suggested, reflects on the person of Jesus on the cross and our reactions to that.
A custom that developed for the liturgy of Tenebrae was the 'strepitus' - this was a loud noise made as the seventh candle was removed and hidden created by banging books on pews and sounding rattles and untuned percussion instruments. The 'strepitus' signifies the confusion and terror which accompanies the death of Christ and his burial. If you think you can pull it off with your pupils, it may be worth trying: maybe a sustained loud and discordant chord from the organ, or rattles and woodblocks sounded by selected pupils. Obviously this needs to work well - if it is likely to descend into laughter and farce it is best left out! But it can be effective, especially as it is followed by silence and dismissal in silence.
If you are producing a printed order of service, you may wish to include a powerful image at the end which pupils can look at as they wait to be dismissed.
There is a PowerPoint to accompany this adaptation of Tenebrae with moving clouds to accompany the introduction, an image for each reading, and a candle being extinguished for the end.