Alison,
A great reference tool that you will need at your disposal for teaching is the "Standard Symbols for Welding, Brazing, and Nondestructive Examination" by AWS Publication A2.4-98. This book covers the definitions of the parts of the welding symbols and discusses the difference in the arrow side and opposite side.
As far as your question referring to the arrow, Here is what the AWS A2.4 has to say:
Paragraph 3.2.1 "Arrow side. Welds on the arrow side of the joint shall be specified by placing the weld symbol below the reference line."
Paragraph 3.2.2 "Other Side. Welds on the other side of the joint shall be specified by placing the weld symbol above the reference line."
Paragraph 1.5 "Placement of the Welding Symbol. The arrow of the welding symbol shall point to a line on the drawing which conclusively identifies the proposed joint. It is recommended that the arrow point to a solid line(object line, visible line); however, the arrow may point to a dashed line(invisible, hidden line)."
As far as I know there is no reference to whether the arrow points up or down, merely that it points to a joint to be welded. I was a Structural Steel Detailer for several years before I got into NDT and Inspection and I have pointed to joints from above the joint pointing down, and below the joint pointing up, and it doesn't change the meaning of the symbol as for what side gets welded. Arrow side and other side, rule as for what side of the joint gets welded.
This book will be your best freind in trying to communicate welding symbols to your students, I highly recommend that you get a copy for your classroom.
Now that I am Inspecting and involved with the welders more, I see lots of welding symbol mistakes on the shop drawings. I know what the detailer has in mind, because I was a detailer, but the symbol he/she may have used is incorrect for the type of joint.
I was re-reading your post and you said you were confused about the arrow use vs the reference line use. I'll take a stab at trying to clarify.
The reference line is related to the joint via the arrow. They are seperate entities, but are the parts of one weld symbol. The arrow points to the joint that the information on the reference line applies to. For example...If I point to a joint that needs a 1/4" fillet weld, welded with SMAW on the near side....I would draw an arrow pointing to the joint with the reference line attached to the arrow, and add a 1/4" fillet weld symbol below the reference line and I may put a tail on the reference line with a welding process(SMAW) indicating that the Shielded Metal Arc Weld welding process is what I want the welder to use to weld the joint. The tail is for additional notes or info to help the welder know what you are trying to convey.
Let me know if I muddied up the water or helped to clear it up,
John Wright