We received many solutions to this problem, including carefully considered solutions from Patrick from Otterbourne School, the Math Club from DMCS in Canada, Siannah from Ricards Lodge School, members of the Pythagorean Club at All Saints Dagenham School, and from Kartik and Mustafa from Wilson's School.
 
Many of you pointed out that the rankings depended on estimates and that different contexts and assumptions might lead to different conclusions (e.g. the size of the bath, the flow of water from a shower, how well the pet rat is looked after, the size of the classroom, the pace at which a leisurely walker travels...).

Bearing this in mind, here is a suggested ranking with the evidence that was provided: 
 
 
Time
 
That a pet rat lives 2-3 years
For all of the red blood cells in your body to be replaced....10 times 3 years
Between two football world cups 4 years
For light to reach us from the nearest star other than the sun 4.2 years

 
Distance
 
You can hop in 5 seconds 8 metres
The distance from the penalty spot to the goal on a football field 11 metres
From the top of a 6 storey building to the ground 18 metres
A car can drive on 1$cm^{3}$ of fuel 20 metres
 
 
Speed
 
Of the fastest sprinter in the world 10m/s
Of a bus going past the school (30mph = 13.4m/s)
Of a racehorse running at maximum speed 15m/s
Of the fastest cyclist in an Olympic cycle sprint race 19m/s
 
 
Mass
 
Of enough potatoes to make chips to feed everyone in the school for a week. (1000kg for 1000 students)
Of a million cubic centimetres of water 1000kg
Of a standard family car 1500kg
Of a team of international male rugby players (15 players) 1500kg

 
Volume of water

used in a 10 minute shower 90 litres
used in 5 dishwasher cycles 100 litres
in a half-filled bath 100 litres
used to flush the toilet 20 times 120 litres (modern toilet) 

Number of

Steps when walking at a leisurely pace for 1 hour 8000
Mars bars with the same total weight as your whole class 20000 (for a class of 30 students weighing about 40kg each)
Stitches in a handknitted jumper 75000
People at a cup final in a large stadium 80000

Area 
 
Of the sheets of paper in a ream (500 sheets) of A4. 30m$^2$
Covered by 5 litres of paint 50m$^2$
Of the floor of the classroom 60m$^2$ 
Of fabric needed to make school uniforms for the whole class will depend on the size of the class, the width of the fabric and the design of the uniform!