Whilst I appreciate that the laptops need replacing my head is not so keen due to us going academy very soon and is not willing to spen on the wireless either unless its proven to work,
The first issue, unfortunately, is going to be the clients here. 802.11g only clients can only hit a max data rate of 54Mbps (actual TCP throughput is about half).
In a rough sense 25 clients on a single 802.11g AP are going to get about 1Mbps each, pretty low.
The fact the clients are dropping in data rate is to be expected in the 802.11 protocols.
When a bunch of clients are fighting for airspace over a small pipe there are going to be collisions and packets loss. When there is packet loss (no ACK) the AP and clients adjust their data rate to a lower. It’s a fail-safe mechanism.
The first step I would do is get the latest support drivers on the WNICS (direct from the vendor..i.e intel.com) and reduce or remove any roaming profiles that may be used.
Second to that, but not least, is to use clients with at least 802.11n 2.4GHz support. Dualband 5GHz clients will be even better.