Your group will be making a model of the solar system that will show the vast distances between the planets in scale.  It will also show the relative sizes of the planets in scale for comparison, BUT you will not use the same scale for both of these.  For the distances you will be using the scale of  1 meter = 1 AU. or about 150,000,000 km. so that the earth will be 1 meter away from the sun.  To show the sizes of the planets in scale, you will make the earth 1 cm.  and the other planets in that same scale which works out to be about 1 centimeter = 13,000 km.   The reason you can't very easily make them to the same scale is that the real distances separating the planets in the solar system are so great that if you used the scale of 1 cm. = 13,000 km. for the distance, your little 1 cm. earth model would have to be 11,538 cm. or over 115 m. away from the sun!  That's more than the length of a football field!  That means Jupiter would be more than five football fields away and tiny Pluto would be only 2 millimeters across and be over 39 football fields away.  For another perspective on this here is a quote from "The Nine Planets" on the Internet:

"One way to help visualize the relative sizes in the solar system is to imagine a model in which it is reduced in size by a factor of a billion (1e9). Then the Earth is about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40000 km away."

These represent the planets lined up close together in their correct order and approximate relative size:

Directions for Your Scale Model

          Using the numbers below you will construct a scale model of the solar system by taping or gluing  little pictures of the planets that you make and color on either adding machine paper from a roll, yarn or string depending upon what your teacher has available.  In addition, will need a meter stick and a metric ruler for your group plus paper and colored pencils and reference books or the Internet to get pictures of what the planets look like.   Your teacher may give you drawing compasses to make the circles.
          The numbers that you will use were obtained by comparing the other planets diameters with that of earth to get a ratio.  It's done like this - take the diameter of Mercury and divide it by the diameter of Earth  (4880 km.÷ 12,760 km.= 0.38) - so that if Earth is 1 cm. then Mercury would be 0.38 cm. or 3.8 mm. across in your scale model which is not very big!  The astronomers have already done something similar with the planetary distances from the sun.  They invented a unit called an AU or Astronomical Unit to simplify the distances by making 1 AU equal to the distance from the Earth to the Sun which means that you can use the standard accepted distances in AU for your distances on you scale.  Just make each AU be equal to a meter!  At any rate the math has been done for you in the chart below so you can concentrate on making your model.
 

      Remember the AU measurements will be converted into meters so your last planet will be 39.5 meters away from the sun on your tape or string.   You don't have to make the sun, just assume it is at one end of the tape.   The dia./ earth ratio will be converted into centimeters so when Earth is made to be 1 cm.across, then Saturn should be 9.5 cm. across.
Just for reference the Sun should be 109 cm. across in this scale because it's diameter is 109 times that of earth.  Actually all the planets could be lined up three times across the sun's middle it is so big!
      One more tip, try hard to keep your tape or string untangled while you work.  Keep it loosely wound or rolled between work sessions.