why did Ethiopia not get grabbed up immediately by European powers?

Ethiopia by the mid 1800s was split up into basically independent tribes/princedoms who would sometimes ally with the Emperor in Gondar and sometimes not when it suited them. It also saw Ethiopia establishing relations with other countries more, this was pronounced in the 1850s-1880s, when both European powers and Egypt were in the region, either through missionary work, diplomacy, trade or conflict; Britain would send a force in 1867 which would put an end to the reign of Teodros II, who started to bring more centralization, and Ethiopia and Egypt would fight in 1875. The British didn't seek control over Ethiopia though, and Ethiopia under Yohannes IV, who would die to the Mahdists and then Menelik II would continue centralization with Menelik building a formidable rifle army. During his reign, most of the rest of Africa was taken by colonial powers, and Italy, thinking that Ethiopia was their protectorate due to their interpretation of a treaty between the powers gave him guns:

For his promise of friendship in 1887, for example, Menilek received 5,000 Remingtons. After Menilek had become Emperor the import of firearms increased enormously. Exact numbers are not easy to obtain, but one early Italian account arrives at a total of 189 thousand firearms from 1885 to 1895.26 So sure were the Italians of their new "protectorate" that they themselves furnished Menilek with arms as gifts as well as with a loan of four million lire, with which Menilek placed orders for arms all over Europe. Arms and ammunition were pouring into Ethiopia particularly through the French port of Obock. By the time the Italians finally grasped the truth that Ethiopia was not their protectorate, Menilek was armed as no Ethiopian ruler before him. He could not only put some 100,000 soldiers in the field, but furnish them all with rifles.[1]

Italy then invaded in 1894 after figuring out this wasn't the case. Ethiopia, with the now strong army that outnumbered the Italian-Eritrean army, along with support from Russia(who was also Orthodox Christian), would defeat Italy at the very famous battle of Adwa, before signing a truce a few weeks later. Ethiopia would continue as independent until Fascist Italy successfully invaded again in 1935.

PS I'm a history teacher looking for new topics to teach and considering Ethiopia as a course so any suggested periods to study or research would also be appreciated.

Depends how big your course is. Kebra Nagast is pretty interesting if it's just a smaller one, we wrote a paper about that at my school.

[1] Sven Rubenson: Some Aspects of the Survival of Ethiopian Independence in the Period of the Scramble for Africa, p 22