The troubleshooting steps below are assuming that you have already ruled out the possibility of a global wifi network outage and have come to the conclusion that the wifi issue is only happening to this one particular mac.
Here are some things you can try if your mac is not connecting to the wifi. If you have several different wifi networks available to connect to, see if you can connect to any other network besides the one that is currently failing. Example, if you are trying to connect to the HUSD_Staff wifi and it is failing, for troubleshooting purposes only, see if can connect the user to HUSD_Wireless or the guest wifi. If you are still having no luck here are some things you can try.
1. Make sure your Mac have has the most recent Mac OS installed and all the system updates. (10.11 El Capitan especially had a ton of wifi issues). If your mac can not install the most recent Mac OS due to hardware limitations, it's time for a new mac! :)
2. Delete the prefered network name (SSID) from the Apple/system preferences/network/ and select wifi in the left side menu and then click the advanced button. Select the HUSD_Staff name and click the "-" sign to delete. Then click ok and close out of the windows and from the desktop choose the wifi icon in the top right corner near the time clock and try and sign onto HUSD_Staff again. It should prompt you for the password again now that you deleted it from the list.
3. Have your CLS/TSS sign in with their wifi username and password to test. If they are successful, then have them try your client wifi username and password.
4. You can create a new test user profile by going into system preferences, users and groups, click the "+" and create a new test account. Once the account is created, sign into this test account and try the same wifi credentials for HUSD_Staff.
5. You can try and sign into HUSD_Staff on another mac. You may need to forget the network on this other mac as someone else may be signed into HUSD_Staff already. You can go into System Preferences/Network/Wifi/advanced button/wifi tab and select HUSD_Staff and click the "-" minus sign in. Click ok and back out and try and sign into HUSD_Staff
6. Delete for the HUSD_Staff keychain
- Open Keychain access
- search airport network
- select the (name of network that is not working) HUSD_Staff network, control click it and then choose delete
- close keychain access and restart your mac
- try and sign into the HUSD_Staff wifi
7. Reinstall OS - Recovery mode - This does not delete
- Restart mac
- hold down command + R on the keyboard while mac is booting up
- If done correctly, you should get the Recovery main menu "MacOS utilities menu"
- Choose "reinstall OS"
- Follow the on screen instructions
- When complete and the mac is ready, log in and try to connect to the HUSD_Staff wifi