Defining connection speed and maximum number of users

As with most internet services it's always "up to". It's not likely you'll see 150Mbps of sustained performance, but mainly more consistent speeds when there are a lot of users on at the same time. Just about all ISPs use very vague numbers and low numbers. For starts, if they can convince you that you need a fiber optic line from them or a higher package, they win. Using low numbers also allows them some denability if you start seeing issues with more than the recommended users online.

As your speed gets faster and faster these speed test sites become less accurate. Simple because you start hitting limits of the speed test server itself. Most of those speed test servers are likely running on standard 1Gbit ports and are normally under heavy load(10+ test at once). Which causes the network adapter to be maxed out, so it can't deliver your full line speed.

As for the average users, there really isn't a set number. It just depends on what you'll be doing with the connection. the 10+ user number is very low IMO. With that number Comcast is assuming the base usage of each user is 15Mbps(1.875MBps). That's a lot to be using consistently for a single user. 

I guess what we should be asking. Is what are these students doing? Downloading large files? Downloading games from Steam or streaming videos? 

Though, if you are having issues with dropped connections that isn't likely related to your speed or concurrent users. So walk us through your setup. Line from the street or whatever comes in to the modem and goes where?

We have 240 students and 60 staff and faculty. Everyone has a smartphone. All teachers and most of the students have laptops. Some students will also have tablets and game consoles. we have computer lab with 36 PCs,  8 PCs in the library and another 20 throughout administrative offices. There are a dozen or so network connected laser printers. We use Google Cloud print along with Googe Apps for eduication.

I use Untangle as our router/firewall and block social networking, streaming video and gaming sites during the school day, as well as, enforce a quota of 3 GB per device.

In the evenings and on weekends students do everything online from gaming, watching movies on Netflix/Hulu/Amazon/Google, Skyping with friends and family half way around the world, messaging, and streaming music from Spotify/Rdio/Beats/iTunes. Many students from Asia will access content from their home countries, such as streaming video, meassaging, games and search engines.

We have 30 - 802.11n Juniper WiFi access points connected to HP Procurve switches via 1Gbps CAT6 cabling which goes through the Untangle router which has an Intel 6 core processor and 16 GB of RAM. The router is also a WAN balancer and connects to two Netgear cable modems that connect to a trunk connection which runs straight into our server room. 

The router reports a bandwidth utilization between 150 GB to 205 GB a day. 

Am I expecting too much of our cable connection or should it support our needs?

It sounds like you're using your connection the way a business class connection should be being used. I wouldn't neccessarily say you're expecting too much from it at all. Even if you were "overloading" the connection all you'd see would be slow speeds, timeouts..etc. 

As for the connection dropping. What is exactly dropping? Are WiFi clients getting dropped? Do you notice any orange or blank lights on the modem during these drops? The next time it happens I'd recommend accessing your modem's informations page and pulling the stats; http://10.1.10.1/ and posting the output from the "Cable modem" page.

One thing you may want to do is run a continous PingPlotter over the course of a few days to a week, so you can graph the problem. While it's not likely to help solve the problem. It's helpful to have it graphed out in front of you and can help you find patterns for troubleshooting. For the best results, I recommed running these test while directly plugged into the modem. 

Thanks for your responses Kraze.

By "dropping" I mean that Google Docs will say, "Lost connection to server, reconnecting." Skype calls will get disconencted. Web pages will refuse to load and just spin.  In the eveings students wil say every 20 minutes or so the connection drops.

At first I thought it migt be wifi coverage, but this will even happen on wired connections as well. I am usualy not near the modem when this occurs to notice the lights on the modem. When I looked at the information pages on both modems yesterday downstream power was within -4 to +4. SNR is above 30 and the upstream power is lower than 50 db, within 47-49 db. 

I have run PingPlotter for days on both a PC behind the router and another directly connected to one of the cable modems. Lost packets most frequently occur during the heaviest usage times. The addition of the second modem along with stricter enforcement of quotas noticeably reduced the number of dropped packets.

(Forgive me, I didn't mean to select your post as a solution)

Your upstream power level is right on the border of being out of spec. It's possible at certain times it's flucutating out of spec and causing issues. I'd call Comcast and ask that they examine the modem levels and see if they notice any issues. If they don't notice the power level mention it to them (We can also confirm this if you're able to grab the output during the issue). 

If at all possible you should try to observe the modem during these drops. It'd be helpful in determining if your connection is being stressed or if Comcast is to blame here. As for the PingPlotters, what did they specifically show? Was the modem showing high latency or loss?