What standard do you use b, g, n, a ?
If you can tell what equipment do you use ? Is it some SOHO device like Dlink/Tplink etc. ?
1) For example, if you have 1 AP standard B/G in the room and it is broadcasting their SSID (it is open for all),
it is possible that you will have some problems with WiFi.
You write that there are 15 laptops in this room, when all will talk in the same time,
sending/receiving data with maximum speed,( maybe some laptops update the system etc.) network will slow down.
Remember also that most people have smartphones and the devices could have Wifi card enabled.
In this moment all devices will connect to this only one AP. (more than 15)
Have you any log how many devices was connected in the same time to this AP?
2) Maybe someone put their phone in Router/AP mode on the same channel as AP ?
Then you will have interferences.
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Jarek