disable WordPress 404 for one specific page/folder to receive actual php errors

You can add a RewriteRule to your .htaccess to instruct mod_rewrite to stop processing any URI that begins with Staff/ or resolves to Staff:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule ^/?Staff(/|$) - [END,NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

In effect, this should be the same as a RewriteEngine Off directive in a .htaccess file in /Staff.

Breaking down the new rule:
RewriteRule ^/?Staff(/|$) - [END,NC]

• ^/?Staff(/|$): The first argument of a RewriteRule is a REGEX pattern to check against the path portion of requested URIs, in this case Staff/Schedule/Schedu-Mod_Results_test.php
  • ^ matches the very beginning of the URI path
  • /? matches whether or not a leading forward-slash is present (this is only necessary to compensate for old Apache servers – it isn’t needed in Apache 2+
  • Staff matches the literal string Staff
  • (/|$) matches either a forward-slash, or the very end of the URI path
• -: If the pattern is found in the URI path, the second argument specifies what to replace the entire URI path with. In this case - indicates that the URI should not be modified.
• [END,NC]: the third argument consists of boolean flags that further modify the rule.
  • END is similar to L (last), however indicates that this should be the very last rule processed within the context of the directory: if the pattern matches the URI, don’t process any subsequent rewrite rules OR rounds (thus eliminating any possibility of handing the request to WordPress)
  • NC (NoCase) makes pattern matching case-insensative.