Figure 1. Study Design, Measures, and Data Collection.

Student and school characteristics (characteristics of the school community, operational features of the school, and broader context), home environment, and friendships may influence students’ mental health, mental well-being, and adjustment to lockdown and return to school directly or indirectly through different mechanisms.¹² SEL indicates social-emotional learning; SEND, special educational needs or disabilities.

^aThe other racial and ethnic groups category includes Arab, Asian, Black/African/Caribbean, mixed/multiple ethnic groups, and other ethnic groups. This category was grouped together owing to small sample sizes.

^bThe time point 3 (T3) to time point 4 (T4) window for cohort 1 was a mean (SD) of 364.6 (50.9) days and for cohort 2 was 523.9 (47.4) days. The UK government announced on March 23, 2020, that residents must stay at home and some businesses had to close. This lockdown was gradually eased until July 4, 2020, when most businesses were allowed to open. From late July 2020 onward, a series of limited and local restrictions were put in place. From November 5, 2020, the UK went into a second, 4-week lockdown, which differed from the first in that schools and universities remained open, and from December 2020 onward returned to a system of local restrictions. From December 26, 2020, the UK went into a third lockdown with a gradual end: in March 2021, schools reopened; in April 2021, individuals in groups of up to 6 were allowed to meet again outdoors, and outdoor pubs, restaurants, and nonessential shops reopened in the UK; in mid-May 2021, outdoor social contact became unregulated again, individuals in groups of up to 6 were allowed to meet indoors, and restaurants and hotels could reopen. See eFigure 1 in Supplement 1 for a more detailed description of the pandemic timeline and lockdown restrictions at the T4 measurement wave for cohort 2.