The art and design teacher can invigilate the art exam. However, if they are required to give extensive technical support during the exam then an additional invigilator should be used. Please see section 12.4 of JCQ ICE for more information regarding this:
12.4 The timed Art examination will normally be invigilated by an Art and Design teacher. However, as the Art and Design teacher may be required to give technical assistance to an individual candidate, additional invigilators should be deployed at the head of centre’s discretion to ensure the supervision of candidates is maintained at all times.