One disadvantage is that the sample is unlikely to be representative of all students. Snowball sampling involves finding one participant and asking them to recommend others. When investigating students' experiences of streaming, a student is likely to recommend their friends, who are probably in the same stream and share similar attitudes towards school. This means the researcher may end up with a sample of students from only one stream (e.g., all from an anti-school subculture in a bottom set), and their findings cannot be generalised to the experiences of all students in the school.
Snowball sampling is a non-probability sampling technique where existing study subjects recruit future subjects from among their acquaintances. It is often used when the target population is hard to access (e.g., criminal gangs). The main disadvantage is that it is highly unlikely to produce a representative sample. The sample is self-selecting and biased towards the social networks of the initial contacts. In a school context, this would likely result in a sample of students who are all friends with each other, probably from the same stream and with similar views, thus failing to capture the diversity of experiences across different streams.
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