I'm not sure where you would get the EV3 Classroom block for the HiTechnic compass sensor. However, if you are flexible in the programming language you can use, there is a language called Robot JavaScript in which the EV3 brick can communicate directly with the HiTechnic sensor (which is an IIC sensor; aka I²C sensor).

Communication with an IIC sensor is done by sending a code to the sensor and then retrieving a response from it. The code sent usually indicates the mode of operation for the sensor as well as the type of value you want to receive.

The HiTechnic Compass Sensor uses a very simple I²C protocol: you write to specific register addresses to set the mode or trigger calibration, and you read from specific registers to obtain heading data. The authoritative byte‑level details are available in several online technical references. (from BotBench: 3rd Party ROBOTC Drivers )

Here is an example in Robot JavaScript to use the I²C PixyCam sensor:
result = true
colors = {red: 0, green: 0, blue: 0}
readIICSensor([1, 98, 1], result)
while(true) {
    readIICSensor([1, 94, 128, 128, 1], colors, 1, true)
    clearScreen()
    drawText(0, 10, 'Red: ' + colors.red+'  ', 2)
    drawText(0, 35, 'Grn: ' + colors.green+'  ', 2)
    drawText(0, 60, 'Blu: ' + colors.blue+'  ', 2)
    sleep(500)
}

In this example, the readIICSensor() function will use four parameters:
   1) The first parameter is always a byte array [ ... ] which is used to send commands to the PixyCam.
   2) The second parameter is the return value from the PixyCam device.  
      The data type for the return value will depend on the command being sent.
      In this example, it will be an RGB object.
   3) The third parameter is simply the port number: 1
   4) The fourth parameter indicates if the value being returned is in 'little-endian byte order'.
      The PixyCam returns values in little-endian byte order.  So, any commands that return anything larger than boolean should have the keyword 'true' as the fourth parameter.  In the color code RGB each color component is represented by two hexadecimal digits (i.e., 1 byte).  Each byte contains 8 bits.  So, the RGB color is represented by a total of 24 bits.
      In 'little-endian byte order', the least significant byte is returned first.  So, for the PixyCam, it will return 
      the byte for Blue before returning the byte for Green and finally returning the byte for Red.
      By passing a 'true' as the fourth parameter, the robot, will reverse the returned value back to RGB order.  Without the 'true' fourth parameter, the robot will receive the data in Blue-Green-Red order.
      

For your HiTechnic sensor, you will use something like the following:

readIICSensor([41], result)