Author's note: Avatar the Last Airbender and all it's characters belongs to Nick, Michael DiMartino and Bryan Konietzko. I am just borrowing them for a bit. All pairing are canon and this follows canon events.
Also, I have no beta so all mistakes are mine, if anyone would like to be my beta that would be wonderful
100 years ago, the start of the war
The meeting was unprecedented. Never before have the heads of the Air Temples met in such large numbers but the times demanded it; Twelve years after the death of Avatar Roku there were whispers of an attack on the nations by Fire Lord Sozin. The hope of the world fell upon the Air Nomads because they had the Avatar. Traditionally they would have waited until Aang was sixteen to tell him of his identity but they did not have the luxury of time. The boy had to train in all the elements as soon as possible, just in case. After much debate, some of quite heated, the monks of the Northern Air Temple agreed to tell the young novice of his destiny. The possible evacuation of the novices was also debated because everyone was on edge and the monks and nuns wished to be prepared. They hoped that they would never have to evacuate the Air Temples.
They were wrong.
A few weeks before the arrival of Sozin's comet the whispers of an attack became more persistent and the heads of the Air Temples met again. They decided that they now had no choice to evacuate the younger novices; they would leave the option of going up to the older novices. The Avatar was gone and they now had no choice. The majority of the monks of the Northern Air Temple presumed that Avatar Aang was dead but Monk Gyatso held out hope until the end. The head of the temples assembled all its inhabitants and told the novices of the plan. The younger novices and any of the older novices who wished to would leave the temples in the days to come and go into hiding with the help of the Air Nomads that lived outside as well allies within the Earth Kingdom. When the danger had passed the monks and nuns would go and collect them and they could continue with life as always. Unfortunately that was not the case; the Fire Nation invaded the Air Temples and killed all the inhabitants in search for the Avatar. The Fire Nation's army did not find the Avatar and in the wake of the Fire Lord's anger he ordered his army to kill any Airbender they came across and to kill any found hiding them.
The Fire Nation did not know that the head of the Air Temples had sent away their young novices; they assumed that all the novices resided in the temples as they have done since the beginning. It was not common knowledge that the novices had escaped the massacre of the Air Temples. No one looked for the novices because they were all assumed to be dead and because the only ones that mattered were Airbending masters. The novices, like their culture and holy leaders, slipped out of living memories.