I’m a big believer that parents have to figure out what works for their family and their kids. If this works, great.
I’m also of the mind that just carrots or just sticks won’t do it for most kids – you need carrots ($ in this case) and sticks (removal of privileges).
I think the idea that it’s too long of a period is a valid one. Perhaps you can arrange with the guidance counselor to get monthly status updates.
I was the “bright but lazy” student – and that’s what my dad did – if the monthly report showed I was underperforming he cracked down – I could do NOTHING but homework and extra studying. One month of that and I was back on track.
At the time I thought my dad was the meanest dad on the planet – in hindsight, I know he was doing it because he cared and wanted me to achieve my potential.
But – if paying for grades works – great. What is your backup plan if it doesn’t work?
