The SEND Code of Practice (paragraph 6.32) says children may have a wide range of social and emotional difficulties. They may become withdrawn or isolated, or show challenging, disruptive or disturbing behaviour. These behaviours may reflect underlying difficulties such as anxiety, depression, self-harm, substance misuse or eating disorders, or conditions such as ADHD or attachment disorder. Schools should have clear processes to support these pupils (paragraph 6.33).
SEMH replaced the older category "behavioural, emotional and social difficulties" when the Code was introduced in 2014. The change reflects the idea that behaviour is often a sign of an unmet need rather than the need itself.
The safeguarding link. KCSIE 2026 says mental health problems can, in some cases, be a sign that a child has suffered or is at risk of abuse, neglect or exploitation, and can develop into safeguarding concerns such as self-harm or suicidal thoughts (paragraph 227). Staff shouldn't try to diagnose, but they can spot concerns early and pass them on (paragraph 228). Most schools have a mental health lead, and many have a Mental Health Support Team (paragraphs 230 to 232).
Common confusion: a child can have SEMH needs without being on the SEN register. Whether they need SEN support depends on whether they need provision that is additional to or different from what other pupils get.