I am far from a Watson Glaser professional but I shall share my two pence (partly as practice!)
WARNING - this lengthy response might be entirely wrong; if so, I apologise in advance and would appreciate any corrections.
Statements -
When approaching the statements, I like to start by (1.) doing a venn diagram and/or (2.) taking the contrapositive.
With this statement, you learn two things: (1.) there's a class (aka group) of people called 'members of a diplomatic service'. (2.) all intelligent pupils are part of that class. This is a good statement to do a venn diagram for - big circle is the class - 'members of a diplomatic service'; within this circle, draw a circle to represent members - 'all intelligent pupils'.
With this, I like taking the contrapositive - this is a fancy way of flipping the statement back-to-front, then negating whatever was positive and vice-versa. It's a good thing to do - I definitely encourage having a look into it if you haven't. The contrapositive would be - "All intelligent pupils are not normal pupils". This is your learning number three.
Putting these three 'learnings' together, you can now attack the conclusions pretty quickly because you've got a better understanding.
Proposed Conclusion 1 - No member of a diplomatic service is a normal pupil.
Here, I like to look at learning 1+2 (or just statement 1) - the only thing you know is that intelligent people ARE part of the class of 'members of a diplomatic service'. Now, we do know that per learning 3, intelligent students are not normal. BUT - there is nothing stating that intelligent people are the ONLY people who are part of the diplomatic service; we know nothing about who else might be part of that class. In other words, the class is not specified as being exclusive to intelligent pupils - there may well be be normal people + intelligent pupils. Therefore, the proposed conclusion DOES NOT follow.
Proposed Conclusion 2 - Every intelligent pupil takes an active role in the service.
Immediately, one word should jump out - 'active'. Even if the sentence is true regardless of that adjective, the word is not used in the statements therefore we know nothing about it. Therefore, DOES NOT follow.
Proposed Conclusion 3 - The normal pupil cannot be an active member of a diplomatic service.
See my response to Proposed Conclusion 2
Proposed Conclusion 4 - Some members of a diplomatic service are abnormal pupils.
Look at Statement 1 again. We know that all intelligent pupils are members of a diplomatic service. We also know, from taking the contrapositive of Statement 2, that all intelligent pupils are not normal. Therefore, since all intelligent pupils are abnormal + all intelligent pupils are members of the diplomatic service, we can say that conclusion FOLLOWS.
Fun thing to remember about the Watson Glaser - words like 'some' do not have their everyday meaning. When we normally say 'some', we just mean a portion of a whole. In the Watson Glaser, 'some' can mean everything from 1 to everything. So, even if the only people who comprised the diplomatic service were intelligent pupils, proposed conclusion 4 would still follow, given that 'some' can mean 'all'.
