Full-time teachers who are timetabled for 50% or more of their teaching periods in educable mentally handicapped classes and/or occupational entrance classes and/or trainable mentally handicapped classes, and Resource Teachers, and who have a Special Education Certificate acceptable to the Department of Education shall receive $813.00 above salary as provided in Article 4.01. This clause shall not apply to any teacher who comes on staff after January 1, 1981.