What you said
If you are part-time, after-school detentions might be difficult. Why not stick to break or lunchtime detentions? Kids often hate them more because they're losing time with pals so will do more to avoid them.
Stay calm, speak softly and plug away at them assertively, not aggressively. Be determined to raise the standard of discipline and they will thank you for it and start to enjoy your lessons. In the meantime, punish them for their cheek.
The expert view
Sounds like you are doing all the right things: consistency, praise.
As you are part-time, be sure that every detention is logged and enlist someone else to take detentions when you are away. Your head of department would be the obvious candidate. Or go around the hierarchy until someone will.
If detentions slip through the net, or someone no-shows and the follow-up does not happen, pupils just think, "Brilliant, we might get away with it." It ends up being as bad as if you didn't have any sanctions or boundaries.
A child bragging that "You're the only one who gives me detentions", is not something for them to be proud of. Other teachers may not bother, but they should. And it is their behaviour in your lessons that counts here.
Plan lessons for behaviour - get stuck in with worksheets if you have to - behaviour is prior to learning. Set them something the nice ones can do by themselves, leaving you to get the others on task, taking names as appropriate. Make sure you have a seating plan. And find time for phone calls.
You are a good teacher. You care and want them to do well. Be kind to yourself.
Tom Bennett's latest book, Teacher, is published by Continuum. Post your questions at www.tes.co.ukbehaviour.