Why and how did you enter these competitions. Did your school encourage or help you or did you completely do this on your own. Do you think it is important, did it help or do you regret the time spent.
Obviously intel and siemens comes from research. However, on here I see references to other national USA competitions that I have never heard of.
D’s school does not encourage students to enter competitions. Not that they actively discourage it or anything, it is just that no one tells the kids, ok the national X qualifying exam is this week, if you win you go to the state competition. No one mentors them and encourages them to submit a paper to anything.
While they have certain teams like a debate team or a math team, they rarely make it past the local round.
What are younger D’s options if she is interested in competing in something? Is there a central source of information for competitions, I would imagine it goes by area.
Interesting questions. I learned when my D was in 8th grade (she is now a senior) that the onus is often on the student or the parent to find these opportunities. In 8th grade, my D had a wonderful English teacher who encouraged her students to apply to the National Scholastic Awards, a prestigious national competition. My D entered and received an Honorable mention. Several classmates also got mentions or placed. Then she entered HS and it was never brought up again. Because I remembered it, I encouraged my D to take part again, but she didn’t. She thought it was too much work, and then just lost interest. In her 8th grade class though, it was an assignment, which made it easy for the kids to enter.
Our school will occasionally send emails to alert kids of competitions. They always promote the Engineer Girl competition. The sci research program definitely has the student participate in local and regional comps.
There are ways to find out what competitions are going on. I did this for a while, trying to interest my kid. I seem to recall just googling “writing competitions” or similar. There are definitely websites that will tell you about various competitions, and I recall finding a school website that was great, providing links to various competitions.
Basically, tell your child to start searching, there are MANY competitions out there. If your school participates in Model UN or Mock Trial, that should be set up for them by the teacher advisor, and those are great clubs. But yes, if you kid wants to enter competitions, he/she needs to find them for the most part. And you could probably send an email to your kid’s counselor at school to ask if the know of any.
My kid self studied for the USA Biology Olympiad. Her bio teacher helped with signup, we paid the fee, and he proctored her tests. She also tried the NACLO linguistics competition, although it did not turn out to be something she was good at. 
http://nacloweb.org/
The National Scholastic Awards are another one to look at. Watch the deadlines, there is stuff required by a teacher and I seem to remember that the deadlines fall over winter break.
There is something called History Bowl. My kid’s Quiz Bowl coach organized for them to go – but I think the competition is individuals, they just competed individually if I remember right.
She didn’t really do this stuff for her admissions profile – she is just a kid with a lot of academic interests who wasn’t always challenged enough at her high school.
We looked for stuff on our own, and sometimes the teachers will recommend competitions to the girls. Sometimes they’ll enter if it looks cool, sometimes they won’t because it is too big of a time crunch for them and they can’t fit it in comfortably.
At this point where they are in HS I don’t help them search, if I hear something I’ll mention it to them, but it’s really their thing to do or not now. I used to have a big pinterest board with all the possible competitions they could enter when they were in middle school, but that’s because I love creating boards more than they wanted to enter all of them.
My younger one (15) wants to get to the state competition in robotics just because the competition looks fun, not for the bennies. It cracks me up that she’s more enthused about the whole spectacle of it vs. the advantages it could confer, but that’s who she is.
Here’s one that’s popular at our high school for those future neuroscience majors: https://www.sfn.org/public-outreach/education-programs/brain-bee
And this stuff is totally word-of-mouth. The school doesn’t provide info, although a teacher who is attuned to a particular competition might mention it, especially if someone did well in prior years.
Our HS was like the OP’s in that entering competitions (outside of athletics) was very ad hoc. Most activities like mock trial, Model UN, Robotics, Speech & Debate, etc. were considered clubs. Clubs needed a faculty sponsor and preferably an active parent booster club to be effective. Faculty sponsors varied greatly as to how effective they were as did the parent booster clubs. .
If there’s a particular EC that your kid is interested in, I’d focus on that EC and try to figure out how to enter competitions. As an individual, keep an eye out for essay competitions.
Not only watch your own school’s guidance department website, but check out the guidance dept. websites from schools in your area (including private schools) that are well regarded. I found out about one local award/scholarship by basically stalking another school’s website to see what their students were competing in. Also private schools usually have more robust, well funded extracurriculars so you can often learn a lot from checking out what they’re doing.