Hi thereI regularly use the "Pass Trinity" series books for my Trinity students.
The Grades 7-8 one is great as a base to teach your student the basics for the exam but I would recommend that you use additional materials such as magazines and newspapers as well in order to augment their vocabulary for the different subject areas.
If you use it as a base you will be sure that you have covered the neccesary grammar and vocabulary for the exam...but other materials make your lessons more interesting!
Remember that the "Interactive Task" is quite difficult to "teach" - you need to use lots of dialogue situations with your student, teach him/her to naturally ask questions as they would in their mother tongue during a conversation.
Best of luck!
Regards
Carole ("vincenza"), native speaker teacher, Italy.
