He says the number of students in technical studies classes can drop from about 60 in S3 to 10 in S5 Higher Still groups.
The 4 Learning television series Design Solutions, four 15-minute programmes for Intermediate 2 and Higher technology students, launched this term, should help.
Mr Johnston, who is a past president of the Technical Teachers Association, was involved in initial planning for the series and is enthusiastic about it. "Ideally we would like to get our students out into industry to see design and production at first hand, but that isn't practical, so this is an ideal alternative."
The programmes, broadcast on Tuesday mornings, deal with graphics, electronics, product design, systems and control. Young designers at Scottish companies talk about various design processes.
Susan McLaren, who is a trained architect and a lecturer in technology at Strathclyde University, wrote the teachers' notes and says the series gives teachers "an important window" into areas of design. She believes pupils should be encouraged to take a holistic view of design processes, assessing the impact of a product or building on people, the environment and even its place in history.
Duncan Lamont, a senior lecturer in graphic design at Cardonald College, says pupils who have their eye on graphic design courses are often advised to focus on graphic communication at Standard or Higher level but should be encouraged to gain good qualifications in art instead.
Graphic design requires strong aesthetic understanding and artistic skills more than the mechanical drawing skills taught in graphic communication.
Potential graphic designers of websites and corporate logos should be producing top quality portfolios with a strong design focus.
Mr Lamont wants guidance and careers teachers to gain a better understanding of this and give pupils clearer advice. Those with strong graphic communication skills might be encouraged to study 3D design instead. Furniture designers need a combination of good technical drawing skills and artistic talent, he says.
Design Solutions, last programme October 14, Channel 4, 9.30-9.45am