Ah, it all depends on WHICH record you’re trying to break. Last year the workshop guys at my company built the biggest one ever (i think 3.4 meters), and it did fly but not really that high, but that didn’t matter cause it was only about size.
So this summer the world record isn’t for who goes the farthers, or highest, it’s a group effort of most rockets in the air at once. So a few hundred people around the UK (and a few others in other countries) will need to synchronize themselves, launch all the rockets at once, and they send it all the proof (pictures, video) to the Guiness book fo world records people so they can approve the record.
As far as how to build the rocket, there are a million guides and videos online, but the best thing is to try things out. if i win and send your school the rockets we’ll give some instructuions too on how to build a really good one 😉