your population is the entire students of 10th class of 158 school sampled.

I think that you selected randomly 10 schools out of 158 schools. Than no point to think about total universe. I call your FGD as Focus Group Discussion, you again select pupil randomly, if universe very large, but if universe is Small you involve all. Actually, in FGD, we record information through participatory observation source /method. In particular about number is not necessary or important.

Sibgha -

I am not certain that I completely understand the question, but it sounds like the population (for each question/variable) is desired data from all 10th grade students in 158 schools (the 10th grade students in the 158 schools comprise the universe), and the sample is to be data taken as a subset of 10th grade students from 10 of those schools.  This indicates the need for two-stage sampling.  "Cluster sampling," "two-stage sampling," "subsampling," and "multistage sampling" are terms you can research.  Three textbooks I recommend which cover this and other survey sampling topics are:

Thompson, S.K.(2012), Sampling, 3rd ed, John Wiley & Sons.

Cochran, W.G(1977), Sampling Techniques, 3rd ed., John Wiley & Sons. 

Lohr, S.L(2010), Sampling: Design and Analysis, 2nd ed., Brooks/Cole.

But also, it appears that it may be flawed that you already know you want 10 schools.  Consider that the primary sampling units are schools, and you draw students from within these selected schools in a second stage.  (See primary units and second-stage units as described, for example, on page 274 in Cochran(1977).  I think it's easy to forget which is labeled 'primary,' as I just did myself.)  The unit of interest, I assume, is the student, the second-stage unit.  But the number of students in your sample is dependent upon how many schools are drawn and how many students are drawn from each school.  Further the number drawn from each school might vary by school size, as one option.  Your goal is to collect data from the smallest number of students in total which can give you acceptable accuracy for key variables, which depends upon design, and standard deviations.  You might have guessed a good number of schools to use and secondary unit draws by having first done a pilot application, but I am guessing your choice of 10 schools was arbitrary or forced upon you by logistic constraints.  Such constraints are often the reason for cluster sampling.  But if your choice was arbitrary, you may want to reconsider as you learn more about the data. 

Best wishes - Jim

PS - Selections are to use random sampling, but if you are talking about focus groups, rather than individual responses, then that does change things. However, it seems that perhaps the organization of stages may still be a useful concept.

If this is about focus groups or something similar, then sorry, I do not know any references for that. Sorry I missed that when reading and trying to understand the question.

In principle your population is all of 10th grade in all 158 schools.

How valid your sample is depends on how you chose it.

If you randomly selected your 10 schools and then randomly selected the 6 students, that is probably fine. But that does presuppose that all your schools are roughly similar. If for instance there are big differences between schools - for instance urban and rural areas and all your randomly selected schools came from the same area there could be questions asked and it might be better to initially use a stratified sample - and randomly select schools within certain categories.

You also need to be careful about selecting students. Random sample would be best and certainly be wary of taking the easy option of only including students who volunteer or who are nominated by the school.

normally 25% of the population becomes your sample that would be all the students of Grade 10 from at least 40 schools . So in case if there are 20 students per class that will make 800 students which is a sufficient sample size and should give a good KMO

depends on the number of variables to consider, at least I see: (1) .. different schools (n different per course) .. therefore you can be from different cities (2), you may be interested in sex (3)

Consider at least stratifying by city or social stratum and by school

at least it is a Bietapico sampling, STRATUS by school, Simple Random within the classroom.

But it depends on the number of variables that could have differentiating effect on the answer, for example the sex of the student, and this depends on the theory on the basis of your research

Thanks all for you valuable views. I will consider all the views while sampling .

You are badly messed up.you took something between a stratified and a cluster sample and chose the students incorrectly. Why didn't you look at a sampling book to see how this is actually done so you could get usable results. Everything you need is in Cochran, Sampling Techniques, 3rd ed or Schaeffer, mendenhall and ott, Elementary Survey Sampling 3rd,4th,etc,ed.

As it stands any data collected is useless. You need to repeat your sampling to get anything useful. Don't they have a faculty at your university?. D. Booth