Many students are unable to accurately represent a magnetic field using field lines.
Misconception
By midsecondary level, many students recognise a ‘magnetic field’ as a region where magnetic effects can be detected, but many are unable to use field lines to represent the field around common shapes of magnet, or a wire or simple coil.
Resources to Address This

Representing Magnetic Fields: In Practice (1114)
Source  SPT/ Em01 PN11
This resource discusses some important physics narrative for teachers about representing magnetic fields in the classroom.
View Resource 
Drawing Magnetic Field Patterns (1114)
Source  SPT/ Em01 TA04
Remember that some care needs to be taken in leading pupils to draw magnetic field patterns. Rather than getting the pupils to start with iron filings and make their own drawings, we would suggest starting with a compass needle demonstration, which helps pupils to understand and recognise what it is that they are drawing, and then progressing to drawing the iron filings.
View Resource 
Drawing magnetic field patterns  an activity (1416)
Source  SPT/ Em01 TL06
Our experience over many years of directing pupils to scatter iron filings around bar magnets has shown that the magnetic field patterns which seem so clear to us are far from obvious to the vast majority of pupils. Indeed how else could it be? This is one of those situations where:
… if you know what you are looking for, it's obvious … if you don't know what you are looking for, it's hopeless!
The aim of this activity is to develop the idea of a magnetic field by engaging pupils in drawing some magnetic field patterns.
View Resource 
Episode 411: Describing Magnetic Fields (1619)
The field around a permanent magnet should be familiar to your students. In practice, where we want a controllable field, we use electromagnets. In this resource, aimed at ALevel classes, students learn about these fields and the factors that determine their strength and direction.
View Resource 
Magnetic fields due to arrangements of magnets (1116)
Source  Practical physics/ Electromagnetism/ Permanent magnets/ ...
Iron filings will align with the direction of a magnetic field to reveal the field pattern.
View Resource
References
The following studies have documented this student thinking:
 Borges, A. T. and Gilbert, J. K. () Models of magnetism. International Journal of Science Education, 20 (3),
361378.
 Bradamante, F. and Viennot, L. () Mapping Gravitational and Magnetic Fields with Children 9–11: Relevance, difficulties and prospects. International Journal of Science Education, 29 (3),
349372.
 Guth, J. () An indepth study of two individual students' understanding of electric and magnetic fields. Research in Science Education, 25 (4),
479490.
 Saarelainen, M.; Laaksonen, A. and Hirvonen, P. E. () Students' initial knowledge of electric and magnetic fields—more profound explanations and reasoning models for undesired conceptions. European Journal of Physics, 28 (1),
51.