Did the Intel® Centrino® Advanced-N 6235 come already installed in these laptops or was it installed after they were bought? If they came already installed, the drivers must be installed directly from the Stone support page as our drivers found in our support page are generic and may not fit the specifications for your laptop system as the OEM (Original Equipment Manufacturer) adapts it to fit their system. Please take a closer look at the links below as the information in them may help you resolve this issue.
1 - How do I improve my 802.11n wireless performance:
2 - Recommended settings:
From Stone* Support Page you can use the driver finder: Driver Finder | Support | Stone Group but you need to enter the serial number of you the laptops in order to help finde the correct drivers. Below is what was found in their knowledge base that can also help you finding the solution.
Please let us know if this information helped you to resolve the issue.