Everyone who sent in answers had to think about how to show the order of each little calculation. Some wrote down the answer after each step, some used the 'rule' that any multiplying and dividing should be done before the adding and subtracting, and some put brackets around bits that had to be done first.
All of the calculations shown below lead to 100 - but you could think about whether they have chosen the best way to write their answers.

Rayner from Tao Nan School, Singapore sent in these two:
1 x 2 x 3 + 4 + 5 + 6 + 7 + 8 x 9 = 100
123 + 45 - 67 + 8 - 9 = 100

Christopher from Tattingstone School
First of all I put down the numbers 1 to 9 on a bit of paper and kept adding, multiplying, dividing and subtracting until I got the right answer and these are what I came up with.
9 x 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 100
7 x 9 x 2 - 6 -5 - 4 - 3 - 8 x 1 = 100

Rowena 1 + 2 + 3 + 4 +5 x 6 - 7 + 8 + 9 =100
I just tried some sums until I found one that worked.

Helena from Tattingstone School
7 x 8 Ã· 4 x 6 + 1 + 3 + 5 + 9 - 2 = 100

Sally from Tattingstone School
1 x 2 + 3 x 4 x 5 - 6 + 7 + 8 - 9 = 100

Gina and Kate from Yarm Primary School, Stockton-on-Tees

\begin{matrix} 9 x 5 = & 45 \\ + 8 = & 53 \\ x 2 = & 106 \\ - 6 = & 100 \\ - 7 = & 93 \\ + 3 = & 96 \\ + 4 = & 100 \\ x 1 = 100 \end{matrix}

James from Yarm Primary School, Stockton-on-Tees

\begin{matrix} 9 x 8 = & 72 \\ + 7 = & 79 \\ + 6 = & 85 \\ + 5 = & 90 \\ + 4 = & 94 \\ + 3 = & 97 \\ + 2 = & 99 \\ + 1 = & 100 . \end{matrix}

Jessica from Tattingstone School, UK ... I played around with the numbers ... until I had 500 which I could divide by 5.

\begin{matrix} 9 x 8 x 7 = & 504 \\ + 3 = & 507 \\ + 4 = & 511 \\ + 1 = & 512 \\ - 6 x 2 = & 500 \\ Ă \cdot 5 = 100 \end{matrix}

Jason from Priory School, UK
(1 x 2) + (3 x 4) x 5 +(6 + 7 + 8 + 9) = 100

Alexander from Crofton Junior School, Kent
[(1 + 2) Ã· 3] + 4 + (5 x 6) + (7 x 8) + 9 = 100

\begin{matrix} 9 x 8 = & 72 \\ + 1 + 2 + 3 + 4 + 5 + 6 + 7 = 100 \end{matrix}

Daniel from Anglo-Chinese School, Singapore
(1 + 2 + 3 + 4) x (((-5 + 6) x (-7 + 8)) + 9)
= 10 x (((-5 + 6) x (-7 + 8)) + 9)
= 10 x ((1 x (-7 + 8)) + 9)
= 10 x ((1 x 1) + 9)
= 10 x (1 x 9)
= 10 x 10
= 100

Lennox from Norbury Manor Primary School
(9x8) + (4x6) + 9 + 1 + 5 + 2 + 3 = 56 + 24 + 20 = 100

Declan and Harry from Norbury Manor Primary School
(7 x 8) + (4 x 6) + 9 + 1 + 5 + 2 + 3 = 56 + 24 + 20 = 100