How should I budget for these classes?
Choose lessons that rely on disposable or recyclable materials and save your $ for what you cannot avoid obtaining by any other means. I will also buy materials, if it will save me a considerable amount of time or if it will keep kids from fighting. Some materials can be shared, and others should not. I go to the dollar store in Berkeley and SCRAP in San Francisco. I visit the garage sales I see on Nextdoor, and I use social media to ask my friends for donations. I plan most lessons around natural, freely available materials such as sea glass and eucalyptus seeds.

I want to reuse art supplies to make multiple projects and avoid waste. 
This noble goal is beyond the scope of this Q and A, but if you have already chosen the type of lesson you want to do I might be able to help you individually. In general, it is best to plan a lesson around a material that is plentiful, disposable and free, such as cardboard, plastic pieces, donated yarn, old books, etc.

How do you you plan meaningful lessons they can take something away from?
A lot of this is covered in Five Great Principles. But you can also involve the students’ cultures, if you feel confident and knowledgeable in them. Or, you can ask them for input on topics they care about (“What if we made a sculpture of Sonic the Hedgehog?” “What’s the deal with this neighborhood? How can we represent it?”). The use of exit slips is also tied to better planning because you learn more about your students and that info helps you make adjustments. The elements above, and giving them enough time to work on something, will ensure they will care more about what they make. If your students put it on display or take it home, you will have achieved your goal.

How much activity is realistic for one day?
Not as much as you think. You can get ahead of it by observing your students, because your first teaching experiences with that group will show you what your students can do. To give yourself breathing room, choose activities that can be completed by different types of individuals, in flexible amounts of time. For example, a mosaic-making activity is an open activity because it is very adjustable. It may be done freeform or made to fit increasingly intricate patterns or shapes.

Some of my students had to leave early. What now?
Even if you’ve been told you have 45 minutes, your lesson will have to be shorter, say 30 min. Even adults leave early for one reason or another, so plan a shorter lesson and some supplemental activities for those who are left.

My students finished early and I didn’t have anything planned for them!
Teens who finish early can get an art extension activity. Here are some tips for elementary level students who finish early. For middle school students, I like to bring different art-making books that I let them browse or use. Themed drawing instruction books (“Learn To Draw Manga”) are very popular. Or you can have a simple, ongoing project that they pick up if they finish early, for example, weavings or mosaics. For very young students, I try to avoid this as much as possible by organizing the lesson in stations. Scroll down for resources on how to do this. Adults who finished early can be asked to help clean up.

I feel awful I planned a lesson that was too demanding.
Everyone plans lessons like that sometimes. Don’t panic. The key is to acknowledge the feelings this might generate. If working with adults or teens, be honest right away. Don’t let anyone struggle for longer than necessary. Lay out your plan B: “Uh folks, We have fifteen minutes left and I just realized I should have explained how to connect the wires to the armature. I am so sorry. Let’s take the rest of our time to go over this, and we’ll finish next time.”

If working with kids, stop the presses. If several are struggling because the activity is too hard, use a focusing tool to get their attention then redirect them to a different area. Then take the responsibility for how things are going, so that they don’t think the reason they can’t do it is that there is something wrong with them. Tell them you made a mistake, and that you are going to change things around so that they can finish what they are doing. Then give them their new instructions.