If you are looking for some problems to use for Mathematics Team Challenges in school or just for class activities with a difference which will help develop mathematical skills there are some excellent sources of materials.
The United Kingdom Mathematics Trust have annual team challenges for junior : UK years 8 and 9 (ages 12-14, I have also successfully used these with Year 10) and senior: UK years 11, 12 and 13 (ages 15-18). UKMT publish materials free online for use in school, these can of course be used for all students and make an excellent class activity for individuals or groups. The junior materials can be found here and senior here. I have found the crossnumber puzzles an excellent activity for the end of term, these are crosswords with a difference as lots of the clues depend on other clues! Which clues to solve first?! All materials needed and full instructions are provided. Note the Supervisor’s booklet with answers and instructions; this includes the complete crossword grid and clues. I have used the Group competition and relay race problems very successfully in class.
Another great source is Math-Team-Matics – a competition from Grand Valley State University, you can see problems here so you could try any of these activities in class. The individual test has multiple choice questions. I like the look of the 2013 team challenge – a combination of staircases and painted cubes with surface area thrown in as well- I’ll definitely be using that!