I've planned a few of those trips - and because my wife is a teacher - I've actually been on a few. If you and your wife were able to travel to Europe for 7-10 days and together spend less than $2500 ($750 each for flights and $100/nt hotel), then, by your example, the school trip was three times that amount or more than $7500 per student? That's a lot of bake sales and car washes.
It's also applies to oranges in that the student trips are generally prepaid (but refundable) and all-inclusive. Their trips include flights, hotels, in-Europe transportation (usually a 50-passenger touring bus), meals, guide fees, entry fees, gratuities, and some form of travel protection. Most of the student trips I've experienced are priced generally at - or slightly above - products like Globus or Trafalgar. (Back then, I think our intra-Europe touring had a targeted-budget well under $200 per student, per day - flights were purchased separately at consolidator rates).
That said, this was not my preferred mode of travel! There aren't too many things more stressful than worrying HOURLY about losing a kid in Europe, And, as teenagers, many were determined to break the rules! In addition, because we had to get 30-50 seats on the same plane - based on availability - we got the worst flight times and layovers. Those teenagers are sleeping on their bags in the airport for a reason.
The European touring buses were perhaps the "perceived" luxury item on these trips. Many have video and wifi to keep kids entertained. The hotels were always LARGE and usually not in the city center (easier for the buses to access and far enough out to keep kids from heading to the Spanish Steps). The tours were often groups of 25 (or more), wearing earpieces and following one guide - which was like herding cats. The restaurants were always a fixed menu - exceptions made only for those students with dietary concerns. And at night, there was a curfew. As chaperones, I don't think we ever slept! We devised techniques like tying bells to doors or masking tape to see who broke the seal. So for me, this was NOT my preferred method of exploring Europe.
But for the 17-year old students, it was incredible. They were first-timers in Europe, "on their own" with their buddies, and their "war stories" would far exceed their experiences! For a few years, this would be the GREATEST TRIP OF MY LIFE! I just don't recall too many of these costing SIX TIMES the price of one person traveling to Europe.
Is there a markup versus booking internet promotions independently? Most certainly. Can an educated traveler replicate a touring experience? No doubt, I do it all the time and it appears that you do also. I would say many people who take the time to post their experiences on Travel forums like this probably do too. But I would push back on what that markup is and each person then has to determine the value of the supporting travel group, DMC, travel agency, or guide and will it enhance their experience.
Regarding travel conventions/events... most of the travel events I go to now are located in Europe. You might swing by one on your next trip. For example, the Food Expo in Milan was an incredible experience - like a World's Fair of travel through food. They had more than 22 million visitors. Thus, travel conventions come in many formats.
And I understand your frustration about guides. I've been in the Vatican 250+ times and I cringed every time I heard a "guide" telling a group about Michelangelo having to paint the Sistine Chapel ceiling while lying on his back. The adage "you get what you pay for" certainly applies to guides. If you're on a ticket-entry-only-priced group tour, you're probably not working with a registered guide (Those skip-the-line tours outside the Vatican are an entirely different post). That said, I learned quickly who the best guides were and I'd pay to be on their tours (market research) to improve my presentation.