I’ve read through many chance posts, and those who post seem confident that their recommendations are strong.
I wonder how you can entirely know. My son’s math teacher, for example, was new to the school this year and while my son is very strong in math, would he be able to write a really compelling recommendation?
Suddenly I am second guessing everything we did.
I’ve always wondered as well about all the kids who are so certain of their glowing recommendations. My take is that some see the letters, some are told by their teachers that the recommendations are good, and some are just obnoxiously overconfident. 
Also, don’t forget that for the most part these are the smart kids, and tend to be the kids that aren’t getting into trouble at school. That will skew the recommendations towards the more positive also.
I’d say most of the people who are confident about their recommendations just have really good relationships with their teachers. If you talk to your teacher on a regular basis and you’re pretty sure they don’t hate you, they will probably give a decent recommendation.
I also talked to my extracurricular recommender about the format, and he said that on gateway it was mostly about skill in the subject. I am not sure how it works for math and english, but that is at least how it is set up for extracurriculars.
If you Google “gateway recommendations” you can find PDFs of what your teachers had to fill out.
I’m one of the teachers writing those recommendations. Here’s my spin on it.
For starters, I’m in education because I like kids. I want the best for them, I really do. That desire only stops short at compromising my professional reputation.
For the past 35 years in the classroom, I’ve been asked almost every year to write letters. Some years, like this one, will be delightfully easy, since I’ve only taught about 30 members of the current Junior class, and not all will ask me. Other years, like last year and the one before were brutal, with close to 40 letters per year.
In all those years, I’ve suggested to maybe 2 or 3 kids that perhaps someone else could write a better letter. If I can’t gush about the kid, and I’m confident that someone else can, then I’ll make the suggestion. But for the most part, I can find enough wonderful things to say about the kids I teach.
As each of our kids requests a letter, they’re given a form to fill out. It asks why they chose me, how they did in my class, what they remember most about the class, and I forget what else. Those forms go to their guidance counselor, who hands them to me with my list of letters. They help make the letter writing easier, since I get the kid’s perspective on his time in my class. I also have all my grade books for any kids I’ve taught who haven’t yet graduated. All that helps when it’s time to write the letters.
The fact that a teacher is new to the school won’t effect what he’s able to say about your son’s performance in his class.
As to the kid’s spin on those letters here, take them with a grain of salt. If I were to believe everything I were to read here, the only kids in the USA NOT headed for the Ivy League would be my own 3 kids, and almost all of the kids I teach. I think that overconfidence, and certainly overconfidence when posting online, are part and parcel of being a teenager.
Sure, the kids will get good letters because their teachers take the time and effort to write those good letters, to find that something in every kid that makes his mom and his girlfriend love him so much. It’s not always easy to see, but it’s almost always there.