Hi I tutored DD1 at home - initially with a group of friends, but over the summer one to one.
Advantages of group:
they see that other children are doing the extra work too.
competition can drive them to work hard.
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Disadvantages of group:
Children can find harder work a bit frightening and may act up/ be disruptive (as one did in our homespun group)
Can become play time rather than work time
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advantages working one to one:
Total attention on your child and their strengths/ weaknesses
Can tailor work to your child's needs
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Disadvantages
Can lead to a sense of isolation (I'm the only one doing all this, it's not fair scenario)
Can become combative if the child is resistant to doing the work or being challenged
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Having done a bit of both there's no winning. What I will say is preparing for the 11+ (especially if this is not Y5) is likely to be a long haul - so having friends to do it with at least means you're not on your own.
If you are planning on helping support this and not just being hands-off - then in fact at home you can give the 1 to 1 support, reinforcing group work at the tutorial.
HTH