Drinking is not compulsory. Neither is going to parties. Some people prefer one-to-one, some prefer activities with others that do not involve alcohol.
For a short period in my teens I drank a fair bit with freinds, but it palled , and the sight of my freinds becoming incoherent or at least a bit irrational was not that pleasant when I was the only one sober. Even their idea of what was funny tend to seem a bit pointless.
As I result I picked my occasions and did more one-to-one. By your age parties had disappeared from my life. Perhaps my thoughts might have been a little different from yours but I simply told everyone I wanted to drink a well known soft drink and that was that -still applies today.
If you do want to join them but cannot think how to tell them just invent something, there a whole host of medications for example that prohibit drinking. Adverse reactions to alcohol can be another reason, as can driving. Being a designated driver has its advantages.
In short you are not alone, and your choice is a perfectly acceptable one .