Based on the information provided, Sarah is using alcohol as a way to cope with stress and pressure from school. This behavior could suggest she is beginning to rely on alcohol for relaxation, which could indicate the potential early stages of alcohol-related issues.
The best answer in this context is: the first stage of alcoholism.
It's important to note that just drinking on weekends doesn't necessarily mean someone is an alcoholic; however, if they begin to feel they need alcohol to cope, it's a sign of possible progression toward alcohol dependence.