I’ve been a coasteering guide for 20ish years and have jumped off many a cliff.
Holding your nose doesn’t really work as you’ll probably let go when you hit the water and I’ve seen some good cuts on faces from finger/thumb nails.
Crossing your arms across your chest does work quite well but less so on the higher jumps. 10m/30ft is a reasonable height.
I just tend to jump and land with my arms above my head and blow out of my nose once I’m under water. Doesn’t really work but o quite like the feeling of a good nose flush when it all pours out at a random time later that day.
TL:DR – it’ll be fine; worry more about keeping yer legs together and your body straight; don’t look down as you jump or you’ll get a face smack.
And don’t give in to peer pressure!