For any conclusion to be made on the population based on a sample survey, the sample must be representative of the population. Sample represents the population if the following condition is fulfilled:-
The sample is a Simple Random Sample (SRS). A SRS is chosen in such a way that all possible samples of size n are equally likely. This implies that the sample is not biased.
Getting the representative sample is the challenge. The flaws / conditions ignored by the researchers in this case can be:-
Are only male students surveyed? In that case, the female population is ignored.
Are the students surveyed are from a particular region only? Say students surveyed are from "Alaska" where it is cold in most part of the year and people tend to use less sunscreen.
Are students surveyed are from a particular age group only? Say student surveyed are only from Grade 6. Then the sample does not represent students from other grades.
There are chances that the survey was done at the convenience of the surveyor who approached only those students who were approachable - those playing outside the school. This is called convenience sampling. Though the individuals contacted are easy to contact, they may not be representative of the population.