Easiest- punter
Hardest- Lineman
Difficult to really name a specific position on either side of the line because they all require a certain physicality that is much more advantageous to upperclassmen who are generally more developed and experienced than 13, 14, and 15 yr old freshmen (yes, I've seen ages vary that much if you were wondering).
Sometimes you get the stud 240 lb 6'5" freshman who is gonna be a freak of nature, but I'm guessing most schools don't have one of those. You also rarely see any freshmen starting on either side of the line in most any class, unless their numbers are that low or the kid is just that good.
How many freshmen start on the O or D lines for the top 5 4A schools?
And punter is the easiest because there's a few more things that go into being a kicker, though a lot of it depends on how much a coach invests into those positions. All a punter has to do is catch the ball cleanly and get off even a decent kick in an attempt to change the field position. Most coaches are fine with 30+ yds a kick. 40 yds a kick is usually pretty good for the average HS team.
Kicker is a little more complex, especially if you're gonna do PATs and/or FGs. If they're just for kickoffs, you'd still want a kicker who can get it to the 10-15 yd line at the very least, though I'm seeing more and more kickers these days get very close to kicking it out of the endzones. Usually, freshmen at most schools don't have that kind of leg strength though, but I've seen some that do.