No legitimate campsite will allow an unsupervised group of 13 year olds with one 15 year old to stay on their site.
It's linked to the camp sites' insurance and fire regulations. Legally you're children and the site won't be responsible for a group of unsupervised children in the event of a serious incident.
None , no campsite can get insurance for under 18 without an adult, so you will not be going on your own
Forget Chris's answer, you have to be supervised by an adult. There is also no way you are allowed to camp in woods, it is private property.
None. No campsite will take unsupervised children, it's a legal liability and will invalidate their insurance.
Not going to happen no campsite would have insurance cover
None.
If no-one is 18, then none of you is legally liable, so cannot be held to a contract. Their insurance is unlikely to cover this, in the event they felt like taking the risk.
The best you will get anywhere is to stay without an adult but have an adult arrange it all and leave their contact details.