While I think you'd be better off investigating flex or grid for this (both of which are pure CSS and are well supported now) I appreciate you have said you cannot use them.

One thing that is (even more) widely supported is inline-block.

The given code is simply structured, with the section having 7 parts, of which the first six are to have the same width as each other and the 7th having twice their width.

Using percentages we can divide up the width of the body (or container if one is to be used) to hold 3 such inline divs plus their left and right margins.

This snippet starts the process, but there are things that you will have to decide on further - like what the heights and paddings should be, putting more placeholders in the inputs, what exactly the drop down options should look like, formatting for the submit button, what you want it to do on narrow screens etc. (Note: view the snippet in full screen - then try reducing the width. There comes a point at which you will have to be more careful about allowing for border and padding widths.)

* {
 margin: 0;
 padding: 0;
 box-sizing: border-box; 
}
body {
  background-color: #522464;
}
[class^="webform-component"] {
  display: inline-block;
  margin: 1.5%;
}
.webform-component {
  width: 30%;
}
.webform-component--resource-format {
  width: 60%;
}
label {
  color: white;
  background-color: #522464;
  font-variant: small-caps;
  display: block;
  padding-left: 1em;
}
input, select {
  border: 1px white solid;
  border-radius: 3vmin;
  background: #522464;
  width: 100%;
  height: 3em;
  color: rgba(255, 255, 255, 0.4);
}

<form method="post" name="Form1" action="" onSubmit="return validateForm();" onchange="showForm();" id="form3118" >
  

         <div class="webform-component first-name" >
            <label for="field0" >
              First Name*
            </label>
            <input id="field0" name="firstName" type="text" value="" placeholder="First name"/>
          </p>
        </div>

          <div class="webform-component last-name" >
            <label for="field1">
              Last Name*
            </label>
            <input id="field1" name="lastName" type="text" value="" />
          </p>
        </div>
    

        <div class="webform-component school" >      
            <label for="field2">
              School*
            </label>
            <input id="field2" name="schoolName" type="text" value="" />
        </div>

<div class="webform-component role" >    
            <label for="field3">
              Role*
            </label>
            <select id="field3" name="role">
              <option value="" selected="selected" >
                Please select
              </option>
              <option value="Principal" >
                Principal
              </option>
              <option value="Assistant/Vice Principal" >
                Assistant/Vice Principal
              </option>
              <option value="Curriculum Coordinator" >
                Curriculum Coordinator
              </option>
              <option value="eLearning Coordinator" >
                eLearning Coordinator
              </option>
              <option value="Head of Department" >
                Head of Department
              </option>
              <option value="Teacher" >
                Teacher
              </option>
              <option value="Other school staff member" >
                Other school staff member
              </option>
              <option value="None of the above" >
                None of the above
              </option>
            </select>  
</div>       

        <div class="webform-component  email-address" >          
            <label for="field5" >
              Email Address*
            </label>
            <input id="field5" name="emailAddress" type="text" value="" />
        </div> 


         <div class="webform-component  last-name" >
            <label for="field15">
              Mobile/Phone number*
            </label>
            <input id="field15" name="mobile" type="text" value="" />
          </p>
        </div>

         <div class="webform-component--resource-format" >
            <label for="field13" >
              In what format do you prescribe resources?
            </label>
            <select id="field13" name="resourceFormat" >
              <option value="" selected="selected" >
                Please select...
              </option>
              <option value="Digital only" >
                Digital only
              </option>
              <option value="Digital with some print" >
                Digital with some print
              </option>
              <option value="Equal digital and print" >
                Equal digital and print
              </option>
              <option value="Print with some digital" >
                Print with some digital
              </option>
              <option value="Print only" >
                Print only
              </option>
            </select>
          </div>

     

            <input type="submit" value="Submit"  />
          
  </form>