Phenoxymethylpenicillin oral 500mg 6 hourly (increased if necessary up to 1g 6 hourly) for 10 days 

If true penicillin / beta-lactam allergy:

Doxycycline oral 200mg stat, then 100mg once daily for 10 days

N.B. In pregnant patients, doxycycline is contraindicated. See here for an alternative option in penicillin allergic pregnant patients.

• Scarlet fever is a notifiable infectious disease caused by toxin producing strains of Group A streptococcus (S. pyogenes). Scarlet fever is a clinical diagnosis and throat swabs to identify S. pyogenes may not be helpful, due to poor specificity and sensitivity.
• The primary site of infection is usually the throat and the initial symptoms of scarlet fever are those of streptococcal pharyngitis. A faint red rash, which feels like sand-paper, develops after 12-48 hours. Sparing of the area around the mouth is typical. As the rash fades, the skin of the fingertips, toes and groin peels off. The tongue first shows a white coating, then peels a few days later and looks red and swollen ('strawberry tongue').
• Consider admission for any patient with valvular disease or significant immunocompromise or if you suspect a complication of streptococcal infection, such as acute rheumatic fever or a deep neck space infection. 'Septic' or 'toxic' scarlet fever is associated with invasive Group A streptococcal disease. Patients develop a high fever and marked systemic toxicity, and may have symptoms such as diarrhoea and vomiting, arthralgia and jaundice. This is a medical emergency with a high mortality that should be admitted immediately.
• Advise the patient to stay away from school or work for at least 1 day after starting antibiotic treatment, wash their hands frequently, avoid sharing eating utensils and towels, dispose of handkerchiefs promptly, and avoid contact with anyone at particular risk of infection (e.g. people with valvular disease or who are immunocompromised). Advise to return for follow up if symptoms have not improved or have worsened after 7 days.

Group A Streptococci are universally penicillin susceptible.