It’s quite possible that Reparo would be sufficient to repair Hogwarts. In the game Wonderbook: Book of Spells (which is considered canon), there’s a scene which gives us the history and background of Reparo. There’s a playthrough on YouTube, and I’ve transcribed the relevant portions here:
Many repairing and mending charms have been used over the centuries, but the most effective and powerful was invented by Orabella Nuttley in the eighteenth century. […]
In 1754, two of Europe's most celebrated broom fliers – sworn enemies Torquil MacTavish, of Scotland, Silvio Astolfi of Italy agreed to a public broom race from Aberdeen to Rome. […] Precisly what happened is hotly contested to this day, but a large explosion ensued, and when the smoke cleared Silvio Astolfi had been turned into a chicken, Torquil MacTavish's knees were on back to front and the Colosseum lay in ruins.
For a few horrific minutes, it appeared that the assembled crowd had witnessed the greatest breach the International Statute of Secrecy ever known. Not only were they at a loss to know how to repair such massive damage, but the sound of the explosion had already woken half of Rome.
It was then that Orabella Nuttley came quietly forwards and, with a few waves of her wand reconstructed several columns with the use of her own Repairing Charm. Stunned at its efficacy, the governmental Ministers begged her to teach them how to use the new spell and by the time the first anxious Muggles had dressed and rushed to the scene, they found nothing, but the Colosseum in exactly the state they had last seen it, and a few oddly dressed men trying to soothe a distressed chicken. Orabella received the Order of Merlin, First Class, and her Mending Charm has been in widespread use ever since.
Hogwarts is an order of magnitude larger than the Colosseum, but it seems plausible that you could still use Reparo for a building of this size:
- If you repaired it in Colosseum-sized chunks, rather than all-in-one-go, the problem seems eminently tractable.
- You have more people, who probably know more about the spell (it has an extra 250 years of usage and research since Nuttley’s time), and thus are able to use it more effectively. They probably also have more combined magical power than a single witch.
- It’s quite possible that Hogwarts has some mechanisms for self-repair, or at least to assist in the process. It’s a magical building, not a static one. Depending on the enchantments placed on it by the builders, it might “know” how it’s supposed to fit together, which would presumably assist with the reconstruction process.
Ultimately, I don’t think canon ever tells us exactly how Hogwarts was repaired. I don’t recall any interviews along those lines. But based on the evidence above, it seems that a team of people could probably repair the castle using Reparo alone.