Focus at the homework table is rarely about willpower — it's usually about environment and routine. A few small changes tend to go further than reminders to "concentrate."
Same time, same spot
A consistent homework time and location removes a decision point, so less energy goes into negotiating and more goes into the actual work.
One task visible at a time
A cluttered desk with every subject's books out can be overwhelming. Keeping only the current task in view reduces the urge to jump around.
Built-in breaks
Short, scheduled breaks (not "whenever you feel like it" breaks) actually extend focus over the full session, rather than shortening it.